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철학 - 지혜의 탐구




1 Introduction

Epistemology is one of the main branches of philosophy; its subject matter concerns the nature, origin, scope, and limits of human knowledge. The name is derived from the Greek terms episteme (knowledge) and logos (theory), and accordingly this branch of philosophy is also referred to as the theory of knowledge.


2 Issues of epistemology



Why should there be such a subject as epistemology? Aristotle provided the answer when he said that philosophy begins in wonder, in a kind of puzzlement about things. Nearly all human beings wish to comprehend the world they live in, a world that includes the individual as well as other persons, and most people construct hypotheses of varying degrees of sophistication to help them make sense of that world. No conjectures would be necessary if the world were simple; but its features and events defy easy explanation. The ordinary person is likely to give up somewhere in the process of trying to develop a coherent account of things and to rest content with whatever degree of understanding he has managed to achieve.

Philosophers, in contrast, are struck by, even obsessed by, matters that are not immediately comprehensible. Philosophers are, of course, ordinary persons in all respects except perhaps one. They aim to construct theories about the world and its inhabitants that are consistent, synoptic, true to the facts and that possess explanatory power. They thus carry the process of inquiry further than people generally tend to do, and this is what is meant by saying that they have developed a philosophy about these matters. Epistemologists, in particular, are philosophers whose theories deal with puzzles about the nature, scope, and limits of human knowledge.

Like ordinary persons, epistemologists usually start from the assumption that they have plenty of knowledge about the world and its multifarious features. Yet, as they reflect upon what is presumably known, epistemologists begin to discover that commonly accepted convictions are less secure than originally assumed and that many of man's firmest beliefs are dubious or possibly even chimerical. Such doubts and hesitations are caused by anomalous features of the world that most people notice but tend to minimize or ignore. Epistemologists notice these things too, but, in wondering about them, they come to realize that they provide profound challenges to the knowledge claims that most individuals blithely and unreflectingly accept as true.

What then are these puzzling issues? While there is a vast array of such anomalies and perplexities, which will be discussed below in the section on the history of epistemology, two of these issues will be briefly described in order to illustrate why such difficulties call into question common claims to have knowledge about the world.




2.2.1 "Our knowledge of the external world."

Most people have noticed that vision can play tricks on them. A straight stick put in water looks bent to them, but they know it is not; railroad tracks are seen to be converging in the distance, yet one knows that they are not; the wheels of wagons on a movie screen appear to be going backward, but one knows that they are not; and the pages of English-language books reflected in mirrors cannot be read from left to right, yet one knows that they were printed to be read that way. Each of these phenomena is thus misleading in some way. If human beings were to accept the world as being exactly as it looks, they would be mistaken about how things really are. They would think the stick in water really to be bent, the railway tracks really to be convergent, and the writing on pages really to be reversed. (see also  illusion, appearance)

These are visual anomalies, and they produce the sorts of epistemological disquietudes referred to above. Though they may seem to the ordinary person to be simple problems, not worth serious notice, for those who ponder them they pose difficult questions. For instance, human beings claim to know that the stick is not really bent and the tracks not really convergent. But how do they know that these things are so? (see also  optical illusion, refraction)

Suppose one says that this is known because, when the stick is removed from the water, one can see that it is not bent. But does seeing a straight stick out of water provide a good reason for thinking that it is not bent when seen in water? How does one know that, when the stick is put into the water, it does not bend? Suppose one says that the tracks do not really converge because the train passes over them at that point. How does one know that the wheels on the train do not happen to converge at that point? What justifies opposing some beliefs to others, especially when all of them are based upon what is seen? One sees that the stick in water is bent and also that the stick out of the water is not bent. Why is the stick declared really to be straight; why in effect is priority given to one perception over another? (see also  sense)

One possible response to these queries is that vision is not sufficient to give knowledge of how things are. One needs to correct vision in some other way in order to arrive at the judgment that the stick is really straight and not bent. Suppose a person asserts that his reason for believing the stick in water is not bent is that he can feel it with his hands to be straight when it is in the water. Feeling or touching is a mode of sense perception, although different from vision. What, however, justifies accepting one mode of perception as more accurate than another? After all, there are good reasons for believing that the tactile sense gives rise to misperception in just the way that vision does. If a person chills one hand and warms the other, for example, and inserts both into a tub of water having a uniform medium temperature, the same water will feel warm to the cold hand and cold to the warm hand. Thus, the tactile sense cannot be trusted either and surely cannot by itself be counted on to resolve these difficulties.

Another possible response is that no mode of perception is sufficient to guarantee that one can discover how things are. Thus, it might be affirmed that one needs to correct all modes of perception by some other form of awareness in order to arrive at the judgment, say, that the stick is really straight. Perhaps that other way is the use of reason. But why should reason be accepted as infallible? It also suffers from various liabilities, such as forgetting, misestimating, or jumping to conclusions. And why should one trust reason if its conclusions run counter to those gained through perception, since it is obvious that much of what is known about the world derives from perception?

Clearly there is a network of difficulties here, and one will have to think hard in order to arrive at a clear and defensible explanation of the apparently simple claim that the stick is really straight. A person who accepts the challenge will, in effect, be developing a theory for grappling with the famous problem called "our knowledge of the external world." That problem turns on two issues, namely, whether there is a reality that exists independently of the individual's perception of it--in other words, if the evidence one has for the existence of anything is what one perceives, how can one know that anything exists unperceived?--and, second, how one can know what anything is really like, if the perceptual evidence one has is conflicting.


2.2.2 The "other-minds problem."

The second problem also involves seeing but in a somewhat unusual way. It deals with that which one cannot see, namely the mind of another. Suppose a woman is scheduled to have an operation on her right knee and her surgeon tells her that when she wakes up she will feel a sharp pain in her knee. When she wakes up, she does feel the pain the surgeon alluded to. He can hear her groaning and see certain contortions on her face. But he cannot feel what she is feeling. There is thus a sense in which he cannot know what she knows. What he claims to know, he knows because of what others who have undergone operations tell him they have experienced. But, unless he has had a similar operation, he cannot know what it is that she feels. (see also  sensation)

Indeed, the situation is still more complicated; for, even if the doctor has had such a surgical intervention, he cannot know that what he is feeling after his operation is exactly the same sensation that the woman is feeling. Because each person's sensation is private, the surgeon cannot really know that what the woman is describing as a pain and what he is describing as a pain are really the same thing. For all he knows, she could be referring to a sensation that is wholly different from the one to which he is alluding.

In short, though another person can perceive the physical manifestations the woman exhibits, such as facial grimaces and various sorts of behaviour, it seems that only she can have knowledge of the contents of her mind. If this assessment of the situation is correct, it follows that it is impossible for one person to know what is going on in another person's mind. One can conjecture that a person is experiencing a certain sensation, but one cannot, in a strict sense of the term, know it to be the case.

If this analysis is correct, one can conclude that each human being is inevitably and even in principle cut off from having knowledge of the mind of another. Most people, conditioned by the great advances of modern technology, believe that in principle there is nothing in the world of fact about which science cannot obtain knowledge. But the "other-minds problem" suggests the contrary--namely, that there is a whole domain of private human experience that is resistant to any sort of external inquiry. Thus, one is faced with a profound puzzle, one of whose implications is that there can never be a science of the human mind.


2.2.3 Implications.

These two problems resemble each other in certain ways and differ in others, but both have important implications for epistemology.

First, as the divergent perceptions about the stick indicate, things cannot just be as they appear to be. People believe that the stick which looks bent when it is in the water is really straight, and they also believe that the stick which looks straight when it is out of the water is really straight. But, if the belief that the stick in water is really straight is correct, then it follows that the perception human beings have when they see the stick in water cannot be correct. That particular perception is misleading with respect to the real shape of the stick. Hence, one has to conclude that things are not always as they appear to be.

It is possible to derive a similar conclusion with respect to the mind of another. A person can exhibit all the signs of being in pain, but he may not be. He may be pretending. On the basis of what can be observed, it cannot be known with certitude that he is or that he is not in pain. The way he appears to be may be misleading with respect to the way he actually is. Once again vision can be misleading.

Both problems thus force one to distinguish between the way things appear and the way they really are. This is the famous philosophical distinction between appearance and reality. But, once that distinction is drawn, profound difficulties arise about how to distinguish reality from mere appearance. As will be shown, innumerable theories have been presented by philosophers attempting to answer this question since time immemorial.

Second, there is the question of what is meant by "knowledge." People claim to know that the stick is really straight even when it is half-submerged in water. But, as indicated earlier, if this claim is correct, then knowledge cannot simply be identical with perception. For whatever theory about the nature of knowledge one develops, the theory cannot have as a consequence that knowing something to be the case can sometimes be mistaken or misleading.

Third, even if knowledge is not simply to be identified with perception, there nevertheless must be some important relationship between knowledge and perception. After all, how could one know that the stick is really straight unless under some conditions it looked straight? And sometimes a person who is in pain exhibits that pain by his behaviour; thus there are conditions that genuinely involve the behaviour of pain. But what are those conditions? It seems evident that the knowledge that a stick is straight or that one is in great pain must come from what is seen in certain circumstances: perception must somehow be a fundamental element in the knowledge human beings have. It is evident that one needs a theory to explain what the relationship is--and a theory of this sort, as the history of the subject all too well indicates, is extraordinarily difficult to develop.

The two problems also differ in certain respects. The problem of man's knowledge of the external world raises a unique difficulty that some of the best philosophical minds of the 20th century (among them, Bertrand Russell, H.H. Price, C.D. Broad, and G.E. Moore) spent their careers trying to solve. The perplexity arises with respect to the status of the entity one sees when one sees a bent stick in water. In such a case, there exists an entity--a bent stick in water--that one perceives and that appears to be exactly where the genuinely straight stick is. But clearly it cannot be; for the entity that exists exactly where the straight stick is is the stick itself, an entity that is not bent. Thus, the question arises as to what kind of a thing this bent-stick-in-water is and where it exists.

The responses to these questions have been innumerable, and nearly all of them raise further difficulties. Some theorists have denied that what one sees in such a case is an existent entity at all but have found it difficult to explain why one seems to see such an entity. Still others have suggested that the image seen in such a case is in one's mind and not really in space. But then what is it for something to be in one's mind, where in the mind is it, and why, if it is in the mind, does it appear to be "out there," in space where the stick is? And above all, how does one decide these questions? The various questions posed above only suggest the vast network of difficulties, and in order to straighten out its tangles it becomes indispensable to develop theories.



Philosophy viewed in the broadest possible terms divides into many branches: metaphysics, ethics, aesthetics, logic, philosophy of language, philosophy of mind, philosophy of science, and a gamut of others. Each of these disciplines has its special subject matter: for metaphysics it is the ultimate nature of the world; for ethics, the nature of the good life and how people ideally ought to comport themselves in their relations with others; and for philosophy of science, the methodology and results of scientific activity. Each of these disciplines attempts to arrive at a systematic understanding of the issues that arise in its particular domain. The word systematic is important in this connection, referring, as explained earlier, to the construction of sets of principles or theories that are broad-ranging, consistent, and rationally defensible. In effect, such theories can be regarded as sets of complex claims about the various matters that are under consideration.

Epistemology stands in a close and special relationship to each of these disciplines. Though the various divisions of philosophy differ in their subject matter and often in the approaches taken by philosophers to their characteristic questions, they have one feature in common: the desire to arrive at the truth about that with which they are concerned--say, about the fundamental ingredients of the world or about the nature of the good life for man. If no such claims were asserted, there would be no need for epistemology. But, once theses have been advanced, positions staked out, and theories proposed, the characteristic questions of epistemology inexorably follow. How can one know that any such claim is true? What is the evidence in favour of (or against) it? Can the claim be proven? Virtually all of the branches of philosophy thus give rise to epistemological ponderings.

These ponderings may be described as first-order queries. They in turn inevitably generate others that are, as it were, second-order queries, and which are equally or more troubling. What is it to know something? What counts as evidence for or against a particular theory? What is meant by a proof? Or even, as the Greek Skeptics asked, is human knowledge possible at all, or is human access to the world such that no knowledge and no certitude about it is possible? The answers to these second-order questions also require the construction of theories, and in this respect epistemology is no different from the other branches of philosophy. One can thus define or characterize epistemology as that branch of philosophy which is dedicated to the resolution of such first- and second-order queries.



As indicated above, one of the basic questions of epistemology concerns the nature of knowledge. Philosophers normally interpret this query as a conceptual question, i.e., as an issue about a certain conception or idea or notion called knowledge. The question raises a perplexing methodological issue, namely, how does one go about investigating such conceptual questions? It is frequently assumed, though the matter is controversial, that one can determine what knowledge is if one can understand what the word "knowledge" means, that is, what notion or concept the word "knowledge" expresses or embodies.

Philosophers who proceed in this way draw a distinction between a word and its meaning, and a meaning is generally considered to be the concept which that particular word has or expresses. It is usually further assumed that though concepts are not identical with words, that is, with linguistic expressions, language is the medium in which the meaning of such concepts is displayed or expressed.

The investigation into the nature of knowledge often begins in a similar fashion with the study of the use of the word "knowledge" and of certain cognate expressions and phrases found in everyday language. A survey of such locutions reveals important differences in their uses: one finds such expressions as "know him," "know that," "know how," "know where," "know why," or "know whether." These differences have been explored in detail, especially in the 20th century. The expression "know x," where "x" can be replaced by a proper name, as in "I know Jones" or "He knows Rome," has been taken by some philosophers, notably Bertrand Russell (1872-1970), to be a case of knowledge by acquaintance. Russell thought its characteristic use was to express the kind of knowledge one has when one has first-hand familiarity with a certain object, person, or place. Thus, one could not properly say in the 20th century, "I know Julius Caesar," since this would imply that one had met or was directly acquainted with a person who had died some 2,000 years ago. This sense or use of "know" becomes important in the theory of perception and in sense-data theory, since some philosophers, such as Russell and G.E. Moore (1873-1958), have held that one's awareness of a sense-datum (a notion to be discussed later) is a case of direct acquaintance, whereas one's acquaintance with a physical object, such as a human hand, is not.

The phrases "know that" and "know how" have also played fundamental roles in the theory of knowledge. The British philosopher Gilbert Ryle (1900-76), for instance, argued that "know how" is normally used to refer to a kind of skill that a person has, such as knowing how to swim. One could have such knowledge without being able to explain to another what it is that one knows in such a case, that is, without being able to convey to another the knowledge required for that person to develop the same skill. "Know that," in contrast, does not seem to denote the possession of a skill or aptitude but rather the possession of specific pieces of information, and the person who has knowledge of this sort can generally convey it to others. To know that the Concordat of Worms was signed in the year 1122 would be an example of this sort of knowledge. Ryle has argued that, given these differences, some cases of knowing how cannot be reduced to cases of knowing that and, accordingly, that the kinds of knowledge expressed by these phrases are independent of one another. (see also  operational knowledge)

In general, the philosophical tradition from the Greeks to the present has focused on the kind of knowledge expressed when it is said that someone knows that such and such is the case, e.g., that A knows that snow is white. This sort of knowledge, called propositional knowledge, raises the classical epistemological questions about the truth or falsity of the asserted claim, the evidence for it, and a host of other problems. Among them is the much debated issue of what kind of thing is known when one knows that p, i.e., what counts as a substitution instance of p. The list of such candidates includes beliefs, propositions, statements, sentences, and utterances of sentences. Each has or has had its proponents, and the arguments pro and con are too subtle to be explored here. Two things should, however, be noted in this connection: first, that the issue is closely related to the problem of universals (i.e., whether what is known to be true is an abstract entity, such as a proposition, or whether it is a linguistic expression, such as a sentence or a sentence-token) and, second, that it is agreed by all sides that one cannot have knowledge, in this sense of "knowledge," of that which is not true. One of the necessary conditions for saying that A knows that p is that p must be true, and this condition can therefore be regarded as one of the main elements in any accurate characterization of knowledge.



Mental versus nonmental conceptions of knowledge. Philosophers have asked whether knowledge is a state of mind, i.e., a special kind of awareness of things. That it is has been argued by philosophers since at least the 5th century BC. In The Republic Plato provided the first extensive account of such a view. He regarded knowing as a mental faculty, akin to but different from believing or opining. Contemporary versions of this sort of theory regard knowing as one member of a sequence of mental states that involve increasing certitude. This spectrum would begin with guessing or conjecturing at the lowest end of certitude, would include thinking, believing, and feeling sure as expressing stronger attitudes of conviction, and would end with knowledge as the highest of all these states of mind. Knowledge, in all views of this type, is a form of consciousness, the strongest degree of awareness humans possess, and accordingly it is common for proponents of such views to hold that, if A knows that p, A must be conscious of what he knows. This view is normally expressed by saying that, if A knows that p, A knows that he knows that p.

Many 20th-century philosophers have rejected the notion that knowledge is a mental state. In On Certainty (1969) Ludwig Wittgenstein says: " 'Knowledge' and certainty belong to different categories. They are not two mental states like, say surmising and being sure." But, if knowing is not a mental state, then what is it? These philosophers have accepted the challenge of trying to give a different characterization of what it means to say that a person knows something. They typically begin by pointing out that a person can know that p without knowing that he knows it (a good example is in fact to be found in Plato's Meno, where Socrates gradually elicits from a slave boy geometrical knowledge that the boy was not aware he had). They then proceed to argue that it is a mistake to assimilate cases of knowing to cases of doubting, feeling a pain, or having a certain opinion about something. All of these latter are mental states, and they are such that a person who has such a state is aware that he does.

These philosophers, moreover, typically deny that knowing can be described as being a single thing, such as a state of consciousness. Instead, they claim that one can ascribe knowledge to someone, or to oneself, when certain complex conditions are satisfied, among them certain behavioral conditions. For example, if a person can always give the right answers to questions under test conditions, one would be entitled to say that the person has knowledge of the issues under consideration. Knowing on this account seems tied to the capacity to perform in certain ways under certain standard conditions. Accordingly, though such performances may involve the exercise of intelligence or other mental factors, the attribution of knowledge to someone is not merely the attribution of a certain mental state or state of awareness to that person (as seen in the case of the slave boy in the Meno).

A well-known variant of such a view was advanced by J.L. Austin in his 1946 paper "Other Minds." Austin claimed that, when one says "I know," one is not describing anything, let alone one's psychology or a mental state. Instead, one is engaging in a social act, i.e., one is indicating that one is in the position (has the credentials and the reasons) to assert p in circumstances where it is necessary to resolve a doubt. When these conditions are satisfied, one can correctly be said to know.


2.5.1 Occurrent versus dispositional conceptions of knowledge.

A distinction closely related to the previous one is that between occurrent and dispositional conceptions of knowledge. The difference between occurrences and dispositions can be illustrated with respect to sugar. A sugar cube will dissolve if put into water. One can thus say that, even if the cube is not now dissolving as it sits on the table, it will do so under certain conditions. This propensity to dissolve is what is meant by a disposition, and it is a feature sugar has at all times and in all conditions. It can be contrasted with sugar's actually dissolving when immersed in liquid, which is an occurrence, that is, an event happening at a specific place and at a specific time.

These terms also apply to mental events. One can say of Smith, who is working on a problem, that he has just seen the solution. According to this way of speaking, there is a certain answer that Smith is presently aware of and to which he is attending. In such a case Smith's knowledge is occurrent. But one can also ascribe a different sort of knowledge to Smith. Though Smith is perhaps not now thinking of his home address, he certainly knows it in the sense that, if he were asked, he could produce the correct answer. One can thus have knowledge that one is not aware of at a given moment. One can thus say, as with sugar, that knowledge may be either occurrent or dispositional in character, i.e., that one may or may not be in an immediate state of self-awareness with respect to p, but that in either case it can be said that the person knows that p.

It should be noted that the distinction between dispositional and occurrent knowledge thus applies to cases of "knowing that" as well as to cases of "knowing how" and thus is a powerful conceptual tool for analyzing different sorts of epistemic notions. The concept of a disposition has itself been further analyzed, for example by Roderick M. Chisholm (b. 1916), in counterfactual terms, and it has been proposed by many philosophers that the knowledge expressed by causal laws (laws of nature) is counterfactual and thus dispositional in character.


2.5.2 A priori versus a posteriori knowledge.

A sharp distinction has been drawn since at least the 17th century between two types of knowledge: a priori knowledge and a posteriori knowledge. The distinction plays an especially important role in the philosophies of David Hume (1711-76) and Immanuel Kant (1724-1804). It is also found in many contemporary, empirically oriented theories of knowledge, which typically hold that all knowledge about matters of fact derives from experience and is therefore a posteriori and that in consequence such knowledge is never certain but at most only probable.

The difference between these types of knowledge is easy to illustrate by means of examples. Consider the sentences "All husbands are married" and "All Model-T Fords are black" and assume that both statements are true. But how does one come to know that they are true? In the case of the first, the answer is that, if one thinks about the meaning of the various words in the sentence, one can see that the sentence is true. One can see that this is so because what is meant by "husband" is the same as what is meant by "married male." Thus, by definition, every husband is a married male, and, accordingly, every husband is married. In calling such knowledge a priori, philosophers are pointing out that one does not have to engage in a factual or empirical inquiry in order to determine whether the sentence is true or not. One can know this merely on the basis of reflection and thus prior to or before any investigation of the facts.

In contrast, the second statement can be determined to be true only after such an investigation. One may well know that the Model-T Ford was an automobile built prior to World War II and accordingly would understand what all the words in the sentence mean. Nonetheless, understanding alone would not be sufficient to allow one to determine whether the sentence is true or not. Instead, some kind of empirical investigation is required in order to arrive at such a judgment; the knowledge thus acquired is a posteriori, or knowledge after the fact.

There are sets of distinctions related to the one just developed and in terms of which the two propositions can also be differentiated. They are necessary versus contingent, analytic versus synthetic, tautological versus significant, and logical versus factual. Necessary versus contingent propositions.

A proposition is said to be necessary if it holds (is true) under all possible circumstances or conditions. "All husbands are married" is such a proposition. There are no possible or conceivable conditions under which this statement would not be true (on the assumption, of course, that the words "husband" and "married" are taken to mean what they ordinarily mean). In contrast, "All Model-T Fords are black" holds in some circumstances (those actually obtaining, and that is why the proposition is true), but it is easy to imagine circumstances in which it would not be true--for instance, if somebody painted one of those cars a different colour. To say, therefore, that a proposition is contingent is to say that it holds in some but not in all possible circumstances. Some necessary propositions, such as "All husbands are married" are a priori (though not all are) and most contingent propositions are a posteriori. Analytic versus synthetic propositions.

A proposition is often said to be analytic if the meaning of the predicate term is contained in the meaning of the subject term. Thus, "All husbands are married" is analytic because the term "husband" includes as part of its meaning "being married." A term is said to be synthetic if this is not so. Therefore, "All Model-T Fords are black" is synthetic since the term "black" is not included in the meaning of "Model-T Ford." Some analytic propositions are a priori, and most synthetic propositions are a posteriori. These distinctions were used by Kant to ask one of the most important questions in the history of epistemology, namely, whether a priori synthetic judgments are possible (see below for a discussion of this question). (see also  analytic proposition ) Tautological versus significant propositions.

A proposition is said to be tautological if its constituent terms repeat themselves or if they can be reduced to terms that do, so that the proposition is of the form "a = a." In such a case the proposition is said to be trivial and empty of cognitive import. A proposition is said to be significant if its constituent terms are such that the proposition does provide new information about the world. It is generally agreed that no significant propositions can be derived from tautologies. One of the objections to the ontological argument is that no existential (significant) proposition can be derived from the tautological definition of "God" with which the argument begins. Tautologies are generally known to be true a priori, are necessary, and are analytic; and significant statements are generally a posteriori, contingent, and synthetic. (see also  meaning)

In the ontological argument, for example, God is defined (roughly speaking) as the only perfect being. It is then argued that no being can be perfect unless it exists; therefore, God exists. But, as Hume and Kant pointed out, it is fallacious to derive a factual statement about the existence of God from the definition of God as a perfect being (see the discussion of St. Anselm below). Logical versus factual propositions.

The term "logical" in this connection is used in a wide sense to include a proposition such as "All husbands are married." By analyzing the meaning of its constituent terms one can reduce the proposition to a logical truth, e.g., to "A and B implies A." In contrast, factual propositions, such as "All Model-T Fords are black," have syntactical and semantic structures that differentiate them from any propositions belonging to logic, even in the broad sense mentioned above. The theorems of logic are often a priori (though not always), are always necessary, and are typically analytic. Factual propositions are generally a posteriori, contingent, and synthetic.

These various distinctions are widely appealed to in present-day philosophy. For instance, Saul Kripke (b. 1941) in "Naming and Necessity" (1972) has used these notions in an effort to solve a long-standing problem, namely, how true identity statements can be nontrivial. The problem, first articulated by Gottlob Frege in "On Sense and Reference" (1893) and later independently addressed by Russell, begins with the assumption that the sentences "Scott is Scott" and "Scott is the author of Waverley" are both identity sentences and are true and that the former is trivial while the latter is not. The puzzle arises from the further assumption that any true identity sentence simply says of some object that it is identical with itself. Hence, all such sentences should be trivial. Clearly, however, "Scott is the author of Waverley" is not trivial. But, if it is not, how is this possible?

Kripke argues that all true identity sentences are necessary (i.e., that they hold in all possible worlds) and that some of these, such as "Scott is Scott," are known a priori and accordingly are trivial; but, he argues, some true identity sentences are not known a priori but only a posteriori and are not trivial. In cases of the latter sort, their nontriviality is a function of their being known to be true only after some sort of inquiry or investigation. It is the investigation that provides new information.

A good example would be the following. At one time in human history, ancient peoples did not know that what they called "the evening star" was the same planet called "the morning star." But eventually the Babylonians discovered through astronomical observation that the morning star is the planet Venus as it appears in the morning sky and that the evening star is the planet Venus as it appears in the evening sky. The discovery that these two appearances are appearances of the same object amounted to discovering more than that Venus is Venus. It provided new information, and that is why "the morning star is identical with the evening star" is significant in a way in which "Venus is Venus" is not, even though all of the descriptive terms in both sentences refer to exactly the same object. In similar fashion, the a posteriori finding that it was Scott who wrote Waverley explains the nontriviality of "Scott is the author of Waverley." But no such investigation was needed to determine that Scott is Scott.


2.5.3 Knowledge by acquaintance and knowledge by description.

The distinction between knowledge by acquaintance and knowledge by description was introduced by Bertrand Russell in connection with his celebrated theory of descriptions. Here only the epistemological (as distinct from the logical) version of his theory will be considered. It was invented by Russell to lend support to the basic thesis of empiricism that all knowledge of matters of fact (i.e., all a posteriori knowledge) derives from experience. Russell's program is both reductive and foundationalist. It tries to show that man's system of knowledge is stratified: that some types of knowledge depend on others but that some do not and that the latter form the foundational units which give support to the whole epistemic system. He argued that, because these basic units rest upon direct experience, ultimately all factual knowledge is derivable from experience.

Russell's argument begins with a distinction between two different types of knowledge, that which is and that which is not based on direct experience. Nearly all of man's knowledge is of the latter type. For example, it is known that some 2,000 years ago there lived a Roman statesman named Augustus, that he was the successor to Julius Caesar, who had been assassinated, and that he was a friend of the historian Livy. But, since none of these pieces of information is presently known on the basis of personal experience, what justification is there for calling them instances of knowledge?

Russell argued that information based on direct experience is basic and needs no justification; he called it "knowledge by acquaintance." Information not based on direct experience he called "knowledge by description." One is justified in calling such information knowledge, if one can show that it can be traced back to and thus ultimately rests upon knowledge by acquaintance. To show how this is so in a particular case is to legitimate that particular piece of information as a specimen of knowing. Here is how this reductive process would work in the case of what is known about Augustus.

Whatever information people in the 20th century have about Augustus probably comes to them from literary works, such as Livy's history of Rome. Such information thus comes secondhand, via descriptions in books about the life and activities of Augustus. But why call such descriptions knowledge? The answer is that through a historical process one can trace such information back to an original source like Livy, who was a contemporary of Augustus. One learns, via this process, that Livy in his history of Rome is reporting events that he had witnessed himself or that he had learned from other eyewitnesses. One can call what he tells about Augustus knowledge, because it is testimony that is based upon his or someone else's direct experience. Thus, knowledge by description is a legitimate form of knowledge, even though it is ultimately dependent upon knowledge by acquaintance.

Russell's reductive thesis then was that all legitimate specimens of knowledge are either based upon direct experience or can be shown to be dependent upon such direct experience via a chain of tight historical or causal links. His theory was therefore a form of empiricism, because it tried to show how all knowledge of matters of fact could be derived from experience.

But there is a further feature of the theory, stemming from the empirical tradition of John Locke (1632-1704) and David Hume, that gives a special twist to the notion of "knowledge by acquaintance." According to this tradition, knowledge by acquaintance is always knowledge based upon what Hume called "impressions," or upon what Russell called "sense-data." These for Russell were mental entities that generally, but not always, reflected the characteristics actually possessed by physical objects. But, unlike physical objects, sense-data were the objects directly apprehended in an act of perception. What Russell meant by "direct apprehension" or "direct perception" was itself explicated in terms of the concepts of inference and non-inference. He held that direct perception, i.e., the perceptual awareness of a sense-datum, involves no inference and, accordingly, that knowledge by acquaintance is identical with the perception of sense-data.

The difference between inferential and noninferential perception can be illustrated by an example. Suppose one is working in a room and hears a sound that emanates from an outside source. (Russell considered hearing to be a form of perception.) In such a case the sound is a sense-datum. One need not infer that one is hearing a sound; there is a direct awareness of it. This would be a case of knowledge by acquaintance. On the basis of what one hears in this direct fashion, one might then infer (guess, conjecture, hypothesize) that what is causing the sound is a motorcycle located outside of the room, something that one who is in the room cannot see directly. If one is correct in this supposition, the information obtained in this way would be a case of indirect knowledge. In such a case, one's knowledge that there is a motorcycle in the street is dependent on (and in Russell's sense, reducible to) one's direct awareness of a sound. The example illustrates how indirect knowledge, such as knowledge by description, is derived from direct knowledge, such as knowledge by acquaintance, and in turn how this latter depends upon the direct awareness of sense-data.

It should be mentioned that the distinction between knowledge by acquaintance and knowledge by description can be defended as legitimate and useful independently of a commitment to sense-data theory. In Russell's work the objects of direct awareness are sense-data, but sense-data theory today has few proponents. A philosopher thus might hold that one at least sometimes directly perceives physical objects (which are not sense-data) while accepting that one's knowledge of past events and persons is indirect and is thus knowledge by description.


2.5.4 Description versus justification.

Epistemology during its long history has engaged in two different sorts of tasks. One of these is descriptive in character. It aims to depict accurately certain features of the world, including the contents of the human mind, and to determine whether these should count as specimens of knowledge. A philosophical system with this orientation is, for example, the phenomenology of Edmund Husserl (1859-1938). Husserl's aim was to give an exact description of the notion of intentionality, which he characterized as consisting of a certain kind of "directedness" toward an object. Suppose the object is an ambiguous drawing, such as the duck/rabbit sketch found in the Philosophical Investigations of Ludwig Wittgenstein. A person looking at the sketch is not sure whether it is a drawing of a duck or of a rabbit. Husserl claimed that the light rays reaching the eye from such an ambiguous drawing are identical whether one sees the image of a duck or of a rabbit and that the difference in perception is due to the viewer's structuring of what he sees in the two cases. The theory tries to describe how such structuring takes place, and it ultimately becomes very complex in the account it gives.

In a famous passage in Philosophical Investigations (1953), Wittgenstein states that "explanation must be replaced by description," and much of his work was devoted to carrying out that task, as, for example, in his account of what it is to follow a rule. Another example of descriptive epistemology is found in the writings of such sense-data theorists as Moore, Price, and Russell. They begin with the question of whether there are basic apprehensions of the world, free from any form of inference, and in those cases where they have argued that the answer is yes, they have tried to describe what these are and why they should count as instances of knowledge. Russell's thesis that the whole edifice of knowledge is built up from a foundation composed of ingredients with which human beings are directly acquainted illustrates the close connection between the attempt to characterize various types of knowledge and this descriptive endeavour. The search by some logical positivists, such as Moritz Schlick (1882-1936), Otto Neurath (1882-1945), and A.J. Ayer (b. 1910) for protocol sentences, sentences that describe what is given in experience without inference, is a closely related example of this kind of descriptive practice.

Epistemology has a second function, which, in contrast to the descriptive one, is justificatory or normative. Philosophers concerned with this function start from the fact that all human beings have beliefs about the world, some of which are erroneous and some of which are not. The question to them is how one can justify (defend, support, or provide evidence for) certain sets of beliefs. The question has a normative import since it asks, in effect, what one ideally ought to believe. (In this respect epistemology has close parallels to ethics, where normative questions about how one ought ideally to act are asked.) This approach quickly takes one into the central domains of epistemology. It raises such questions as: Is knowledge identical with justified true belief? Is the relationship between evidence for a belief and the belief itself a probability function? If not, what is it? What indeed is meant by "justification" and what sorts of conditions have to be satisfied before one is entitled to say that a belief or set of beliefs is justified? These two differing aspects of epistemology are not inconsistent and indeed are often found intertwined in the writings of contemporary philosophers.


2.5.5 Knowledge and certainty.

The relationship between knowledge and certainty is complex, and there is considerable disagreement about the matter. Are these concepts the same? If not, how do they differ? Is it possible for someone to know that p without being certain that p? Is it possible for someone to be certain that p without knowing that p? These are the central issues around which the debate revolves. The various answers that have been proffered depend on how the concepts of knowledge and certainty are analyzed. If one holds, for instance, that knowing is not a psychological state but that certainty is, then one would deny that the concepts are identical. But if one holds that knowing represents the highest degree of assurance which humans can obtain with respect to the truth of p, and that such a maximal degree of assurance is a psychological state, one will interpret the concepts to be equivalent. There have been proponents on both sides of this issue.

Further complicating the discussion are subtle distinctions drawn by 20th-century philosophers. For instance, in "Certainty" (1941) G.E. Moore claimed that there are four main types of idioms in which the word "certain" is commonly used: "I feel certain that," "I am certain that," "I know for certain that," and finally "It is certain that." He points out that "I feel certain that p" may be true when p is not true but that there is at least one use of "I know for certain that p" and "It is certain that p" which is such that neither of these sentences can be true unless p is true. Moore argues that it would be self-contradictory to say "I knew for certain that he would come but he didn't," whereas it would not be self-contradictory to say "I felt certain he would come but he didn't." In the former case, the fact that he did not come proves that one did not know that he would come, but, in the latter, the fact that he did not come does not prove that one did not feel certain he would. "I am certain that" differs from "I know for certain that" in allowing the substitution of the word "sure" for the word "certain." One can say "I feel sure (rather than certain)" without a change of meaning, whereas in "I know for certain" or "It is certain that" this substitution is not possible. On the basis of these sorts of considerations Moore contends that "a thing can't be certain unless it is known." He states that this is what distinguishes the word "certain" from the word "true." A thing that nobody knows may well be true, but it cannot possibly be certain. He thus infers that a necessary condition for the truth of "It is certain that p" is that somebody should know that p is true. Moore is therefore one of the philosophers who answers in the negative the question of whether it is possible for p to be certain without being known.

Moore also argues that to say "Someone knows that p is true" cannot be a sufficient condition for "It is certain that p." If it were, it would follow that, in any case in which at least someone did know that p was true, it would always be false for anyone to say "It is not certain that p"; but clearly this is not so. If one person says that it is not certain that Smith is still alive, he is not thereby committing himself to the statement that nobody knows that Smith is still alive: the speaker's statement is consistent with Smith's still being alive, and both he himself and other persons know this. Moore is thus among those philosophers who would answer in the negative the question of whether the concepts of knowledge and certainty are the same. Though it is widely accepted that to affirm that somebody knows that p implies that somebody is certain that p, the case of the slave boy in Plato's Meno seems, at least at first glance, to be a counterinstance. Meno may know, in a dispositional sense, certain theorems of geometry without knowing that he knows, and, if he does not know that he knows, then it would seem that he cannot be certain that he does know. But it has also been argued that, once his disposition to know has been actualized and his knowledge has become occurrent, then, insofar as he does know in this occurrent sense, he is certain of what he knows.

The most radical position on these matters is to be found in Wittgenstein's On Certainty, published posthumously in 1969. Wittgenstein holds that knowledge is radically different from certitude and that neither concept entails the other. It is thus possible to be in a state of knowledge without being certain and to be certain without having knowledge. As he writes: "Instead of 'I know' . . . couldn't Moore have said: 'It stands fast for me that . . .' ? and further: 'It stands fast for me and many others. . . .' " "Standing fast" is one of the terms Wittgenstein uses for certitude and is to be distinguished from knowing. For him certainty is to be identified with acting, not with seeing propositions to be true, the kind of seeing that issues in knowledge. As he says: "Giving grounds, justifying the evidence comes to an end--but the end is not certain propositions striking us immediately as true--i.e., it is not a kind of seeing on our part; it is our acting which lies at the bottom of the language game."



Philosophers not only wish to know what knowledge is but also how it originates. This motivation is based, at least in part, on the supposition that an investigation into the provenance of knowledge can help cast light on its nature. From the time of the Greeks to the present, therefore, one of the major themes of epistemology has been a quest into the sources of knowledge.

Plato's The Republic contains one of the earliest systematic arguments to the effect that sense experience cannot be a source of knowledge. The argument begins with the assertion that ordinary persons have a clear grasp of certain concepts, that of equality, for instance. In other words, people know what it means to say that A and B are equal, no matter what A and B are. But where does such knowledge come from? One may wonder, for instance, whether it is provided by vision and consider the claim that two pieces of wood are of equal length. A close inspection of these pieces of wood, however, shows them to differ slightly, and the more detailed the inspection, via various degrees of magnification, the more disparity one notices. It follows that visual experience cannot be the fount of the concept of equality. Plato applies this result to the operations of all the five senses and concludes that sense experience in general cannot be the origin of such knowledge. It must therefore have another source, which he regards as prenatal (one such account is found in the myth of Er in Book X).

The mathematical example Plato selects to illustrate that the origin of knowledge is not in sense experience is highly significant; indeed it is one of the signs of his perspicacity that he should pick such an example. For, as the subsequent history of philosophy reveals, the strongest case for the notion that at least some knowledge does not derive from sense experience lies in mathematics. Mathematical entities are abstractions--perfect triangles, disembodied surfaces and edges, lines without thickness, and extensionless points--and none of these exists in the physical world, i.e., the world apprehended by the senses. It might be thought that, had Plato selected a different example, say, the colour red, his argument would have been less convincing. But it is a further sign of his genius that he discusses colours as well as mathematical notions and provides good reasons for holding that seeing examples or specimens of red (or any other colour) is not equivalent to knowing what that colour is. Such knowledge must therefore have a different genesis than sense experience.


2.6.1 Innate versus learned.

The puzzle about origins of knowledge has led historically to two different kinds of issues. One of these is the question of whether knowledge (or at least certain kinds of knowledge) is innate, meaning that it is not acquired or learned through experience but in some important sense is present in the human psyche at birth. The matter is still a live issue today, not only in philosophy but also in linguistics and psychology. The linguist Noam Chomsky (b. 1928), for example, has asserted that the "projection phenomenon"--the ability of children to construct sentences that they have never heard before and that are grammatical--is proof of inherent conceptual structures, whereas the experimental psychologist B.F. Skinner (b. 1904) has tried to show that all knowledge is the product of learning through environmental conditioning by means of the processes of reinforcement and reward. (see also  innate idea)

In the extensive historical literature on this topic both the notion of "innateness" and that of "learning" have been given various interpretations. Sometimes, for instance, innateness carries only the sense of a disposition or propensity, but in stronger versions of the thesis, such as Plato's, it is affirmed that humans possess actual pieces of prenatal knowledge. "Learning" also is given a variety of meanings, ranging from trial-and-error methods to inexplicit types of "absorption" of information. There are also a range of "compromise" theories. These typically claim that humans have some knowledge that is innate--the awareness of God, the principles of moral rightness and wrongness, and certain mathematical theorems being favoured examples--whereas other kinds of knowledge--such as knowledge by acquaintance--are gained through experience.


2.6.2 Rationalism versus empiricism.

The second issue that emerges from considerations of the origins of knowledge focuses on the distinction between rationalism and empiricism. Though closely related to the issue of innateness versus learning, the question in this case concerns the nature of the source from which knowledge arises. The history of discussion of the issue indicates that two main sources have been identified and argued for: reason and experience.

Rationalism is the thesis that the ultimate source of knowledge is to be found in human reason. What reason is, in turn, is a difficult question. But, generally speaking, it is assumed that reason is a feature of the human mind that differs not just in degree but in kind from bodily sensations, feelings, and certain psychological attitudes, such as disgust or enthusiasm. For some writers, such as Plato, reason is a faculty, a special facility or structure of the mind. Many later philosophers reject any sort of faculty psychology, and some of them tend to interpret reason in dispositional or behavioral ways. But, whatever the interpretation, a rationalist must hold that reason has a special power for grasping reality. It is the exercise of reason that allows human beings to understand the world they live in. Such a thesis is double-sided: it holds, on the one hand, that reality is in principle knowable and, on the other hand, that there are human, distinctively mental, powers capable of apprehending it. One thus might define rationalism as the theory that there is an isomorphism (a mirroring relationship) between reason and reality which makes it possible for the former to apprehend the latter just as it is. Rationalists affirm that, if such a correspondence were lacking, the effort of human intelligence to understand the world would be impossible.

Empiricism is often defined as the doctrine that all knowledge comes from experience. Almost no philosopher, however, has ever literally held that all knowledge comes from experience. Locke, who is the empiricist par excellence, thought there is some knowledge human beings have--which he calls "trifling ideas" (or trivialities), such as a = a--that does not derive from experience; but he regarded such knowledge as empty of content. Hume held similar views.

Empiricism thus generally allows for a priori knowledge while denigrating its significance, and accordingly it is more accurate to define it as the theory that all knowledge about matters of fact derives from experience. When defined in this way empiricism does represent a significant contrast to rationalism. Rationalists hold that human beings have knowledge about matters of fact which is anterior to experience and yet which does tell them something significant about the world and its various features. Empiricists would deny that this is possible.

The meaning of the term experience is generally limited to the impressions and sensations received by the senses. Thus, knowledge is the information apprehended by the five sense modalities--hearing, seeing, touching, tasting, and smelling. Such knowledge is always about matters of fact, about what one can see, touch, hear, taste, or smell. For strict empiricists this definition has the implication that the human mind is passive -- a tabula rasa, in Locke's idiom; it is an organ that receives impressions and more or less records them as they are. This conception of the mind has seemed counterintuitive to many philosophers, especially those in the Kantian tradition. But it also poses serious challenges for empiricists. For example, it raises the question of how one can have knowledge of items, such as a dragon, that cannot be found in experience.

In response, the classical empiricists such as Locke and Hume have tried to show how the complex concept of a dragon can be reduced to simple concepts (such as wings, the body of a snake, the head of a horse), all of which derive from direct impressions of such items. On such a view the mind is still considered to be primarily passive, but it is conceded that it has some active functions, such as being able to combine simple impressions and ideas into complex ideas. (see also  simple idea)

There are further difficulties: the empiricist must explain how abstract ideas, such as the concept of a perfect triangle, can be reduced to elements apprehended by the senses when no perfect triangles are actually found in nature, and he must also give an account of how general notions are possible. It is obvious that one does not experience "mankind," but only particular individuals, through the senses; yet such general notions are meaningful, and propositions containing these concepts are known to be true. The same difficulty applies to colour concepts. Some empiricists have argued that one arrives at the concept of red, for example, by abstracting from individual items that are red. But the difficulty with this suggestion is that one would not know what to count as an instance of red unless one already had such a concept in mind; and, if that is so, it would seem that experience cannot be the source of the concept. It is generally felt that, despite ingenious attempts by empiricists to deal with such issues, their solutions have not been wholly successful. Indeed, the history of epistemology has to a large extent been a dialectic between rationalism and empiricism in an effort to meet skeptical challenges that are designed to undermine both positions.



Many philosophers past and present and many nonphilosophers who are studying philosophy for the first time have been struck by the seemingly indecisive nature of philosophical argumentation. For every argument, there seems to be a counterargument; and for every position, a counterposition. To a considerable extent skepticism is born of such reflection. Some of the ancient skeptics contended, for example, that all arguments are equally bad and, accordingly, that nothing can be proved. The American philosopher Benson Mates claims to be a modern representative of this tradition, except that he believes all philosophical arguments to be equally good. But he insists that, because they are, they invariably issue in conceptual deadlocks and resolve nothing.

Ironically, skepticism is itself a type of philosophy, and the question has been raised whether it manages to escape its own demurrers. Does it offer arguments, and, if so, are they decisive? The answers to these questions depend on what is meant by skepticism. Historically, the term refers to a complex set of practices taking many different forms--from stating explicit theories to assuming negative attitudes without much propositional content. Thus, it is difficult to define. But, however it is understood, skepticism represents a set of challenges to the claim that human beings do possess or can acquire knowledge.

In giving even this minimal characterization, it is important to emphasize that both dogmatists and skeptics accept a definition of knowledge that implies two things: that, if a person, A, knows that p, then p is true and that, if a person, A, knows that p, then A cannot be mistaken, meaning that it is logically impossible that A could be wrong. If a person says that he knows Smith will arrive at 9:00 AM, and Smith is not there at 9:00 AM, then that person would have to withdraw the claim to know. He might say instead that he thought he knew or that he felt sure. But he could not continue rationally to insist that he knew if what he claimed to know turned out to be false. (see also  propositional knowledge)

It should also be stressed that, given this definition of knowledge, the skeptic does not have to show that A is actually mistaken in claiming to know that p. All he has to show is that it is possible that A might be mistaken. Hence arises the skeptic's practice of searching for a possible counterexample to a claim. If A states that he has had a certain experience, for instance, that of having personally spoken with Smith, who assured him he would keep his appointment at 9:00 AM, then the skeptic can point out that, although one could have such an experience, it is still possible that Smith might not show up; and, if so, A's claim to know is untenable. In effect, by emphasizing the notion of possibility, the skeptic is pointing out that there is a logical gap between the criteria that support the claim and the claim itself. The criteria might be satisfied, and yet the claim might be false; but, if such a possibility exists, the original assertion cannot be a specimen of knowing.

More generally, radical skepticism has tried to show that one might (i.e., could possibly) have all the experiences associated with normal perception or behaviour and yet be wholly mistaken in thinking that these experiences correlate with anything in the external world. For example, a brain in a vat might be programmed by scientists to have the sensation of seeing a tree, even though it is not in fact seeing a tree. Thus, there is a gap between the experience the brain is having and external reality; accordingly, its claim to know on the basis of such a visual experience is mistaken. The skeptic's point is that the disparity between external reality and felt experience is always possible and, accordingly, that knowledge claims based upon such experience cannot be defended.

The ability to find counterexamples explains why skeptics do not challenge but indeed accept the dogmatist's definition of knowledge. That they do so is important because it means that they are not arguing at cross-purposes with their opponents. What they challenge is not the meaning of knowledge but the contention that anybody actually has knowledge in that sense.

Nearly all of the major epistemological theories of philosophy have given rise to skeptical reactions. Many of the greatest thinkers in the Western tradition have assumed that by means of reason or sense experience one can come to have knowledge of reality. But skepticism has challenged the validity of both of these appeals. Skeptics have developed wholesale arguments to undermine the efforts to show that reason and sense experience, which seem to be the only possible candidates, are reliable sources of knowledge. Descartes, for example, considered the hypothesis that an evil genius may delude people into thinking that they are experiencing the real world when they are not. With regard to major epistemological problems, such as the "other-minds problem," the problem of memory, the problem of induction, and the problem of self-knowledge, skeptical doubts have challenged the validity of reason and of sense experience and thus of claims to have knowledge of various aspects of reality. How some of these moves and countermoves actually take place are addressed below. (Av.S.)


3 The history of epistemology




3.1.1 Pre-Socratics.

The central focus of ancient Greek philosophy was its attempt to solve the problem of motion. Many pre-Socratic philosophers thought that no logically coherent account of motion and change could be given. This problem was a concern of metaphysics, not epistemology, however, and in the present context it suffices merely to allude to the arguments of Parmenides and Zeno of Elea against the possibility that anything moves or changes. The consequence of this position for epistemology was that all major Greek philosophers held that knowledge must not itself change or be changeable in any respect. This requirement motivated Parmenides, for example, to hold that thinking is identical with being (what exists or is unchanging) and that it is impossible to think of "nonbeing" or "becoming" (what changes) in any way.


3.1.2 Plato.

Plato (c. 427-347 BC) accepted the Parmenidean constraint on any theory of knowledge that both knowledge and its objects must be unchanging. One consequence of this, as Plato pointed out in Theaetetus, is that knowledge cannot have physical reality as its object. In particular, since sensation and perception have various kinds of motions as their objects, knowledge cannot be the same as sensation or perception. The negative thesis of Plato's epistemology consists, then, in the denial that sense experience can be a source of knowledge on the ground that the objects apprehended through the senses are subject to change. To the extent that humans have knowledge, they attain it by transcending the information provided by the senses in order to discover unchanging objects. But this can be done only by the exercise of reason, and in particular by the application of the dialectical method of inquiry inherited from Socrates.

The Platonic theory of knowledge is thus divided into two parts: a quest first to discover whether there are any unchanging objects and to identify and describe them and second to illustrate how they could be known by the use of reason, that is, via the dialectical method. Plato used various literary devices for illustrating his theory; the most famous of these is the allegory of the cave in Book VII of The Republic. The allegory depicts ordinary people as living locked in a cave, which represents the world of sense-experience; in the cave people see only unreal objects, shadows, or images. But through a painful process, which involves the rejection and overcoming of the familiar sensible world, they begin an ascent out of the cave into reality; this process is the analogue of the application of the dialectical method, which allows one to apprehend unchanging objects and thus acquire knowledge. In the allegory, this upward process, which not everyone is competent to engage in, culminates in the direct vision of the sun, which represents the source of knowledge.

In searching for unchanging objects, Plato begins his quest by pointing out that every faculty in the human mind apprehends a set of unique objects: hearing apprehends sounds but not odours; the sense of smell apprehends odours but not visual images; and so forth. Knowing is also a mental faculty, and therefore there must be objects that it apprehends. These have to be unchanging, whatever they are. Plato's discovery is that there are such entities. Roughly, they are the items denoted by predicate terms in language: such words as "good," "white," or "triangle." To say "This is a triangle" is to attribute a certain property, that of being a triangle, to a certain spatiotemporal object, such as a particular figure drawn on a blackboard. Plato is here distinguishing between specific triangles that can be drawn, sketched, or painted and the common property they share, that of being triangular. Objects of the former kind he calls particulars. They are always located somewhere in the space-time order, that is, in the world of appearance. But such particular things are different from the common property they share. That is, if x is a triangle, and y is a triangle, and z is a triangle, x, y, and z are particulars that share a common property, triangularity. That common property is what Plato calls a "form" or "idea" (not using this latter term in any psychological sense). Unlike particulars, forms do not exist in the space-time order. Moreover, they do not change. They are thus the objects that one must apprehend in order to acquire knowledge.

Similar remarks apply, for example, to goodness, whiteness, or being to the right of. Particular things change; they come into and go out of existence. But whiteness never changes, and neither does triangularity; and, if they do not change, they are not subject to the ravages of time. In that sense, they are eternal.

The use of reason for discovering unchanging forms is exercised in the dialectical method. The method is one of question and answer, designed to elicit a real definition. By a "real definition" is meant a set of necessary and sufficient conditions that exactly delimit a concept. One may, for example, consider the concept of being the brother of Y. This can be explained in terms of the concepts of being male and of being a sibling of Y. These concepts together lay down necessary and sufficient conditions for anything's being a brother. One who grasps these conditions understands precisely what it is to be a brother.

The Republic begins with the use of the dialectical method to discover what justice is. Cephalus proposes the thesis that "justice" means the same as "honesty in word and deed." Socrates searches for and finds a counterexample to this proposal. It is just, he points out, under some conditions, not to tell the truth or to repay debts. If one had borrowed a weapon from an insane person, who then demanded it back in order to kill an innocent person, it would be just to lie to him, stating that one no longer had the weapon. Therefore, "justice" cannot mean the same as "honesty in word" (i.e., telling the truth). By this technique of proposing one definition after another and subjecting each to possible counterexamples, Socrates attempts to find a definition that would be immune to counterexamples. To find such a definition would be to define the concept of justice, and in this way to discover the true nature of justice. In such a case one would be apprehending a form, the common feature that all just things share.

Plato's search for definitions and thereby the nature of forms is a search for knowledge. But how should knowledge in general be defined? In Theaetetus Plato argues that it involves true belief. No one can know what is false. A person may mistakenly believe that he knows something, which is in fact false, but this is only thinking that one knows, not knowing. Thus, a person may confidently assert, "I know that Columbus was the first European to land in North America" and be unaware that other Europeans, including Erik the Red, preceded Columbus. So knowledge is at least true belief, but it must also be something more. Suppose that someone believes there will be an earthquake in September because of a dream he had in April and that there in fact is an earthquake in September, although there is no connection between the dream and the earthquake. That person has a true belief about the earthquake but not knowledge. What the person lacks is a good reason supporting his true belief. In a word, the person lacks justification for it. Thus, in Theaetetus, Plato concludes that knowledge is justified true belief.

Although it is difficult to explain what justification is, most philosophers accepted the Platonic analysis of knowledge as fundamentally correct until 1963, when the American philosopher Edmund L. Gettier produced a counterexample that shook the foundations of epistemology: suppose that Kathy knows Oscar very well and that Oscar is behind her, out of sight, walking across the mall. Further, suppose that in front of her she sees walking toward her someone who looks exactly like Oscar; unbeknownst to her, it is Oscar's twin brother. Kathy forms the belief that Oscar is walking across the mall. Her belief is true, because he is walking across the mall (though she does not see him doing it). And her true belief seems to be justified, because she formed it on the same basis she would have if she had actually seen Oscar walking across the mall. Nonetheless, Kathy does not know that Oscar is walking across the mall, because the justification for her true belief is not the right kind. What her true belief lacks is an appropriate causal connection to its object.


3.1.3 Aristotle.

In Posterior Analytics, Aristotle (384-22 BC) analyzes scientific knowledge in terms of necessary propositions that express causal relations. Such knowledge takes the form of categorical syllogisms, in which the middle term causally and necessarily connects the major and minor terms. For example, because all stars are distant and all distant objects twinkle, it follows that all stars twinkle. That is, the middle term, "distant objects," connects the minor term, "stars," to the major term, "twinkle," in order to yield the conclusion that all stars twinkle. Aristotle, however, recognizes that not all knowledge is provable. Thus, the premises of the most basic syllogisms are known but not provable. In contrast with scientific knowledge, there is opinion, which is not provable and is about what happens to be true but need not be. (see also  science)

Since the knowledge formulated in syllogisms resides in the mind, which is part of or one faculty of the soul, much of what Aristotle says about knowledge is part of his doctrine about the nature of soul and, in particular, human soul. As he uses the term, every living thing, including plant life, has a soul (psyche), a soul being what makes a thing alive. Thus it is important not to equate soul with mind or intellect. The intellect (nous) might variously be described as a power, faculty, part, or aspect of the human soul. It should be stressed that for Aristotle the terms soul (psyche) and intellect (nous) and its constituents were understood to be scientific terms.

Knowledge is something that a person has. Thus it must be in him somewhere, and the location must be his mind or intellect. Yet there can be no knowledge if the knower and the thing known are wholly separate. What then is the relation between the knowledge in the person or his mind and the object of his knowledge? Aristotle's answer is one of his most enigmatic claims. He says, "Actual knowledge is identical with its object."

Here is one suggestion about what Aristotle means. When a person learns something, he acquires something. What he acquires must either be something different from the thing he knows or identical with it. If it is something different, then there is a discrepancy between what he has in mind and the intended object of his knowledge. But such a discrepancy seems to be incompatible with the existence of knowledge. For knowledge, which must be true and accurate, cannot deviate from its object in any way. One cannot know that blue is a colour if the object of that knowledge is something other than that blue is a colour. This idea that knowledge is identical with its object is dimly reflected in the repetition of the variable p in the standard formula about knowledge: S knows that p just in case it is true that p. Although the line of thinking being attributed to Aristotle is defective in several ways, something like it seems to have motivated Aristotle and many other thinkers over the centuries.

To assert that knowledge and its object must be identical raises a question: In what way is knowledge in a person? Suppose that Smith knows Fido. Then Fido is in Smith. Obviously, Fido is not there as he exists in the nonmental world of space and time. In what sense can it be true that a person who knows what a dog is has that object in his mind? Aristotle derives his answer from his general theory of reality. According to him, all (terrestrial) substances are composed of two principles: matter and form. If there are four dogs--Bowser, Fido, Spot, and Spuds--they are the same in some respect and different in some respect. They are the same in that each belongs to the same kind and each functions similarly. Thus, Aristotle reasons, just as Plato had, that there must be something in virtue of which they are the same, and this he calls "form." That is, Bowser, Fido, Spot, and Spuds each have the very same form of being a dog. They are different in that they are made out of different matter, different parcels of stuff. The form that a thing has is more important than its matter because it is the form that makes the thing what it is. If Fido were to lose the form of being a dog and acquire another, he would no longer be the same thing. The stuff out of which Fido is made is not similarly important, and in fact that stuff changes periodically, as body cells change through metabolic processes, without Fido ceasing to be Fido.

To return to the explanation of knowledge, what is in the knower when he knows what dogs are is the form of being a dog minus the matter. According to Aristotle, matter is literally unintelligible and not essential to what Fido or any other dog is; thus its absence is inconsequential for knowledge, though not for Fido.

In his sketchy account of the process of thinking in De anima (On the Soul), Aristotle says that the intellect, like everything else, must have two parts: something analogous to matter and something analogous to form. The first of these is the passive intellect; the second is active intellect, of which Aristotle speaks tersely. "Intellect in this sense is separable, impassible, unmixed, since it is in its essential nature activity. . . . When intellect is set free from its present conditions it appears as just what it is and nothing more: it alone is immortal and eternal . . . and without it nothing thinks."

This part of Aristotle's views about knowledge is an extension of what he says about sensation. According to Aristotle, sensation occurs when the sense organ is stimulated by the sense object, typically through some medium, such as light for vision and air for hearing. This stimulation causes a "sensible species" to be generated in the sense organ itself. This "species" is some sort of representation of the object sensed. As Aristotle describes the process, the sense receives "the form of sensible objects without the matter, just as the wax receives the impression of the signet-ring without the iron or the gold." But, since there are different species for each of the five external senses that Aristotle recognized--sight, hearing, touch, taste, and smell--"species" does not mean "image."


3.1.4 Ancient Skepticism.

After the development of Aristotle's psychology the next significant event for the theory of knowledge was the rise of Skepticism, of which there were at least two kinds. The first, Academic Skepticism, arose in the Academy after Plato's death and was propounded by the Greek philosopher Arcesilaus (c. 315-c. 240 BC), about whom the philosophers Cicero, Sextus Empiricus, and Diogenes Laërtius provide information. Academic Skepticism is also called "dogmatic Skepticism" when it is interpreted as arguing for the thesis that nothing is known. The thesis was inspired by Socrates' avowal that the only thing he knew was that he knew nothing. Thus, it asserts that knowledge is impossible. This form of Skepticism seems to be susceptible to an objection raised by the Stoic Antipater (fl. c. 135 BC) and others that the view is self-contradictory. To know that knowledge is impossible is to know something; hence, dogmatic Skepticism is false.

Carneades (c. 213-129 BC), a member of the Academy, gave a subtle reply. Academic Skepticism, he claimed, should not be interpreted as a claim about how the world is in itself or about a correspondence between thought (or language) and the world, but as a judicial decision. Just as a defendant in a trial does not prove his innocence but relies upon its presumption and defends it against attack, so the Skeptic does not try to prove that he knows nothing but presumes it and defends this presumption against attacks.

Carneades' construal of Academic Skepticism brings it close to the other kind, Pyrrhonism, named after Pyrrho of Elis (c. 365-275 BC). None of his works survive, and scholars rely principally on the early 3rd-century-AD writings of Sextus Empiricus to understand Pyrrhonism. Pyrrhonists assert or deny nothing but lead people to give up making any claims to knowledge. The Pyrrhonist's strategy is to show that, for each proposition with some evidence for it, an opposed proposition has equally good evidence supporting it. These arguments for refuting each side of an issue are called "tropes." For example, the judgment that a tower is round when seen at a distance is contradicted by the judgment that the tower is square when seen up close. The judgment that Providence cares for all things, based upon the orderliness of the heavenly bodies, is opposed by the judgment that many good people suffer misery and many bad people enjoy happiness. The judgment that apples have many properties--shape, colour, taste, and aroma--each of which affects a sense organ, is opposed by the equally good possibility that apples have only one property that affects each sense organ differently.

Pyrrhonists diagnose dogmatism as the unjustifiable preference for one mode of existence over another. Dogmatists prefer wakefulness and sanity over sleep and insanity. But why should sleep and insanity not be the norm? If the dogmatist answers that it is because sleep and insanity involve some deficiency or abnormal physical states, the Skeptic replies, "By what nonquestion-begging criterion are these things said to be deficient or abnormal? Why should insanity not be taken as the primary notion and sanity be defined as the lack of insanity? If it were, then it would not be difficult to see sanity as a deficiency or abnormality, just as insanity currently is. Or why should wakefulness not be seen as the deficient condition in which people do not dream?" The Skeptic does not advocate insanity or sleep but merely argues that a preference for them is no less justified than a preference for sanity and wakefulness.

What is at stake in the preceding Skeptical arguments is "the problem of the criterion," that is, the problem of deciding how one can determine a justifiable standard against which to measure judgments. Truth seems to need a criterion. But every criterion is either groundless or inconclusive. Suppose that something is proffered as a criterion. The Skeptic will ask what proof there is for it. If no proof is offered, the criterion is groundless. If, on the other hand, a proof is produced, a vicious circle begins to close around the dogmatist: What judgment justifies belief in the proof? If there is no judgment, the proof is unsupported; and if there is a judgment, it requires a criterion, which is just what the dogmatist was supposed to have provided in the first place.

If the Skeptic needed to make judgments in order to survive, he would be in trouble. In fact there is another method of survival that bypasses judgment. The Skeptic can live quite nicely, according to Sextus, by following custom and the way things appear to him. In doing this, the Skeptic does not judge the correctness of anything but merely accepts appearances for what they are.

Ancient Pyrrhonism is not strictly an epistemology since it has no theory of knowledge and is content to undermine the dogmatic epistemologies of others, especially of the Stoics and Epicureans. Pyrrho himself was said to have had moral and ethical motives for attacking dogmatists. Being reconciled to not knowing anything, Pyrrho thought, induced serenity (ataraxia).


3.1.5 St. Augustine.

St. Augustine of Hippo (354-430) claimed that human knowledge would be impossible if God did not illumine the human mind and thereby allow it to see, grasp, or understand ideas. There are two components to his theory: ideas and illumination. Ideas as Augustine construed them are the same as Plato's; they are timeless, immutable, and accessible only to the mind, not to the senses. They are indeed in some mysterious way part of God and seen in God. Illumination, the other element of the theory, was for Augustine and his many followers, at least through the 14th century, a technical term, built upon a metaphor. Since the mind is immaterial, it cannot be literally lighted. Yet the entire theory of illumination rested upon the extended visual metaphor, inherited from Plotinus (205-270) and other Neoplatonic sources, of the human mind as an eye that can see when and only when God, the source of light, illumines it. Still, it is a powerful metaphor relied upon even in the 17th century by René Descartes (Discourse on Method; 1637). Varying his metaphor, Augustine sometimes says that the human mind participates in God and even, as in On the Teacher, that Christ illumines the mind by dwelling in it. It is important to emphasize that Augustine's theory of illumination concerns all knowledge, and not specifically mystical or spiritual knowledge. In addition to its historical significance, his theory is interesting for showing how diverse epistemological theories have been. (see also  Middle Ages, Christianity)

Before he articulated this theory in his mature years and soon after his conversion to Christianity, Augustine was concerned to refute the Skepticism of the Academy. In Against the Academicians Augustine claims that, if nothing else, humans know such disjunctive tautologies as that either there is one world or there is not one world and that either the world is finite or it is infinite. Humans also know many propositions that begin with the phrase "It appears to me that," such as "It appears to me that what I perceive is made up of earth and sky, or what appears to be earth and sky." And they know logical (or what he calls "dialectical") propositions, for example, "If there are four elements in the world, there are not five; if there is one sun, there are not two; one and the same soul cannot die and still be immortal; and man cannot at the same time be happy and unhappy."

Many other refutations of Skepticism occur in later works, notably, in On the Free Choice of the Will, On the Trinity, and The City of God. In the latter work Augustine proposes other examples of things about which people are absolutely certain. Again in explicit refutation of the Skeptics of the Academy, Augustine argues that if a person is deceived, then it is certain that he exists. Like Descartes, Augustine puts the point in the first person, "If I am deceived, then I exist" (Si fallor, sum). A variation on this line of reasoning occurs in On the Trinity, when he says that if he is deceived, he is at least certain that he is alive.

Augustine also points out that, since he knows, he knows that he knows; and he notes that this can be reiterated an infinite number of times: If I know that I know that I am alive, then I know that I know that I know that I am alive. This point was codified in 20th-century epistemic logic as the axiom "If X knows that p, then X knows that X knows that p." In The City of God Augustine claims that he knows that he loves: "For neither am I deceived in this, that I love, since in those things which I love I am not deceived." With Skepticism thus refuted, Augustine simply denies that he has ever been able to doubt what he had learned through his sensations or even the testimony of most people.

Skepticism did not recover from Augustine's criticisms for a thousand years; but then it arose again like the phoenix in Egyptian mythology. Augustine's Platonic epistemology dominated the Middle Ages until the mid-13th century, when St. Albertus Magnus (1200-80) and then his student St. Thomas Aquinas developed an alternative to Augustinian illuminationism.




3.2.1 St. Anselm of Canterbury.

The phrase St. Anselm of Canterbury (c. 1033-1109) used to describe his own project, namely, "faith seeking reason" (fides quaerens intellectum), well characterizes medieval philosophy as a whole. All the great medieval philosophers, Christian, Jewish, and Islamic alike, were also theologians. Virtually every object of interest was related to their belief in God, and virtually every solution to every problem, including the problem of knowledge, contained God as an essential part. Anselm himself said that, while true propositions are those that signify what is, ultimately truth is God. This presented Anselm with a problem, which he discusses at the beginning of Proslogium as a prelude to his famous ontological argument for the existence of God. There is a tension between the view that God is truth and intelligibility and the fact that humans have no perception of God. How can there be knowledge of God, he asks, when all knowledge comes through the senses and God, being immaterial, cannot be sensed? His solution is to distinguish between knowing something by being acquainted with it in sensation and knowing something by describing it. Knowledge by description is possible because of the concepts that one forms from sensation. All knowledge about God depends upon the description that he is "the thing than which a greater cannot be conceived." From this premise Anselm argues that humans can know, for example, that God exists, is all-powerful, all-knowing, all-just, all-merciful, and immaterial. Eight hundred years later Bertrand Russell would use the same distinction between knowledge by acquaintance and knowledge by description to develop his influential philosophy, although he would have vigorously denied that the distinction could be employed as Anselm had, namely, to prove that God exists. (see also  religious belief)


3.2.2 St. Thomas Aquinas.

While a Platonic and Augustinian epistemology dominated the early Middle Ages, the translation of Aristotle's On the Soul in the early 13th century had a dramatic effect. Following Aristotle, Thomas Aquinas (1225-74) recognized that there are different kinds of knowledge. Sense knowledge is what results from sensing individual things: thus, one sees a tree, hears the song of an oriole, and tastes or smells a peach. Thomas considered sense knowledge to be low-grade because it has individual things as its object and is also shared with brute animals. Sensation itself does not involve the intellect and is not properly speaking knowledge (scientia). (see also  Aristotelianism)

It is characteristic of scientific knowledge to be universal; the more general in scope a piece of knowledge is, the better. This is not to diminish the importance of specificity. Scientific knowledge should also be rich in detail, and God's knowledge is the most detailed. The detail, however, must be essential to the thing being studied and not peculiar to just some instances of that kind. Although Thomas thought that the highest knowledge humans can possess is knowledge of God, knowledge of physical objects is more attuned to human capabilities, and only that kind of knowledge will be discussed here.

In his discussion of knowledge in Summa theologiae, Thomas Aquinas argues that human beings do not know material objects directly, nor are such things the principal object of knowledge. Knowledge aims at what is universal, while material things are individual and can be known only indirectly. Elaborating on the thought of Aristotle, Thomas claims that the process of thinking that accompanies knowledge consists of the active intellect (intellectus agens) abstracting (abstrahens) a concept from an image (phantasma) received from the senses.

In one of Thomas' accounts of the process, abstraction is the process of isolating the universal elements of an image of a particular object from those elements that are peculiar to the object. For example, from the image of a dog the intellect abstracts the ideas of being alive, being capable of reproduction, movement, and whatever else might be essential to being a dog. All these ideas are common to all dogs because they are essential to them. These ideas can be contrasted with the ideas of being owned by Dion and weighing five pounds, namely, with properties that vary from dog to dog.

As stated earlier, Aristotle typically spoke of a form as being in the intellect of the knower, whereas the matter of an object is unintelligible and remains extramental. While it was necessary for Aristotle to say something like this in order to escape the absurdity of holding that a material object is in the mind in exactly the same way it is in the physical world, there is also something unsatisfying about it. Physical things contain matter as an essential element, and, if their matter is no part of what is known, then it seems that human knowledge is lacking. In order to counter this worry, Thomas revised Aristotle's theory. He said that not the form alone but the species of an object is also in the intellect. A species is a combination of form and "common matter" (materia communis), where common matter is contrasted with individuated matter (materia signata vel individualis), which actually gives bulk to a material object. Common matter is something like a general idea of matter. Since every animal must have a body, it is not enough to conceive of an animal merely as something that is alive. Having flesh and bones, that is, being material, is part of the essence of being an animal. Of course this materiality, which is common to every animal, is not the same as the actual flesh and bone that constitute Fido--hence the distinction between common and individuated matter.

This abstracted species resides in a part of the soul called "the passive intellect," where it is described as being illumined by the active intellect. What this process amounts to is the isolation of those features of the intelligible species that are universal and necessary to it. Thus, to know what a human being is is to have abstracted the ideas of being rational and being capable of sensation, movement, reproduction, and nutrition and to have excluded the ideas of living in a particular place or having a certain appearance, all of which are not essential to being human.

One objection that Thomas anticipated being raised against his theory is that it gives the impression that ideas, not things, are what are known. If knowledge is something that humans have and if what humans have in their intellect is a species of a thing, then it is the species that is known and not the thing. It might seem, then, that Thomas' view is a type of idealism.

Thomas had prepared for this kind of objection in several ways. His insistence that what the knower has in his intellect is a species, which includes matter, is supposed to make what is in the intellect seem more like the object of knowledge than an immaterial Aristotelian form. Also, scientific knowledge does not aim at knowing any individual object but at what is common to all things of a certain sort. In this, Thomas' views are similar to those of 20th-century science. The billiard ball that John Jones drops from his porch is of no direct concern to physics. Even though its laws apply to John Jones's ball, physics is interested in what happens to any object dropped from any height, just as what Thomas says about apples in general also applies to each individual apple.

As assuaging as these considerations might be, they do not blunt the main force of the objection. For this purpose Thomas Aquinas introduced the distinction between what is known and that by which it is known. To specify what is known, say, an individual dog, is to specify the object of knowledge; to specify that by which it is known, say, the phantasm or the species of a dog, is to specify the apparatus of knowledge. The species of something is that by which the thing is known; but it is not itself the object of that knowledge, although it can become an object of knowledge by being reflected upon.

The philosophical optimism of the 13th century dissolved as a consequence of the secular and ecclesiastical condemnations in 1270 and 1277 of certain aspects of Aristotelian philosophy, and worries about Skeptical consequences began to emerge. While the philosophy of Thomas Aquinas was one of the targets of these condemnations, John Duns Scotus was also worried about the Skeptical consequences that could be elicited from the major competitor to Aristotelianism, the Augustinianism of Henry of Ghent (1217-93). According to Henry, God must "illumine" the human intellect on every occasion of its knowing. Not only could no good literal meaning be given to this sense of illumination, but the view also sounds as if all human knowledge were supernatural. Henry's insistence that God's illumination is a natural divine illumination did not persuade many people.


3.2.3 John Duns Scotus.

While he accepted some aspects of Aristotelian abstractionism and also held that there need to be some a priori principles of perception, principles that he attributed to Augustine, John Duns Scotus (c. 1266-1308) did not rest the certainty of human knowledge on either of them. He distinguished four different classes of things that are certain: First, there are things that are knowable simply (simpliciter). These include both true identity statements such as "Cicero is Tully" and propositions, later to be called analytic, such as "Man is rational." According to Duns Scotus, such truths coincide with what makes them true. A consequence of this is that the negation of a simple truth is inconsistent even though it may not be explicitly contradictory. For example, the negation of "The whole is greater than any proper part" is not explicitly contradictory in the way that "Snow is white and snow is not white" is; nonetheless, "The whole is not greater than any proper part" cannot possibly be true and hence is contradictory.

The second class of certainly known propositions consists of things knowable through experience, where "experience" has the Aristotelian sense of something that is encountered numerous times. The knowledge afforded by experience is grounded in the a priori epistemic principle that "whatever occurs in a great many instances by a cause that is not free is the natural effect of that cause." It is important to note that Duns Scotus' pre-Humean confidence in induction did not survive the Middle Ages. The 14th-century philosopher Nicholas of Autrecourt, who has been called "the medieval Hume," argued at length that there is no necessary connection between any two events and that there is no rational justification for the belief in causal relations.

The third class of certainly known propositions consists of things knowable that concern one's own actions (de actibus nostris). Humans know when they are awake immediately and not through any inference; they know with certainty that they think (me intelligere) and that they hear and have other sense experiences. Even if a sense experience is caused by a defective sense organ, it remains true that one is aware of the sensuous content of the sensation: for example, one sees white even if one is mistaken in thinking that the seeing is caused by snow.

The fourth class of certainly known objects consists of things knowable through human senses (per sensus). Duns Scotus said that humans learn about the heavens, the earth, the sea, and all that are in them. This last class of objects that are certainly known things seems to be posited without regard to the threat of Skepticism at all.

Duns Scotus' rendition of intuitive knowledge, however, has the purpose of forestalling the Skeptical move of interposing something between the knower and the thing known that might enable belief to deviate from its object. Intuitive knowledge is indubitable knowledge that something exists. It is knowledge "precisely of a present object [known] as being present and of an existent object [known] as being existent." Further, the object of knowledge must be the cause of the knowledge. If a person sees Socrates before him, then, according to Duns Scotus, he has intuitive knowledge of the proposition that Socrates is white and that Socrates and his whiteness cause that knowledge. Intuitive knowledge contrasts with abstractive knowledge, such as knowledge of universals, for which the object need not be present or even existent. For example, for all one knows from contemplating the nature of dogs or unicorns, they are equally likely or unlikely to exist. (see also  intuition)

It may appear that intuitive knowledge is absolute; either one has it or one does not. But that is not Duns Scotus' doctrine. He held that there is imperfect intuitive knowledge of the past, which is more certain than abstractive knowledge but less certain than present intuitive knowledge. However plausible or implausible this may be, it is worth noting that Russell held the same view but expressed it by using the terms "knowledge by acquaintance" (intuitive cognition) and "knowledge by description" (abstractive cognition).


3.2.4 William of Ockham.

There are several places in Duns Scotus' account where Skeptical challenges can gain a foothold, for example, when he endorses the certainty of sense knowledge and when he holds that intuitive cognition must be of an existent object. William of Ockham (c. 1285-1349?) took his stand against the Skeptical challenge by radically revising Duns Scotus' idea of intuitive cognition. Unlike Duns Scotus, Ockham does not require intuitive knowledge to have an existent object, and the object of intuitive knowledge need not be its cause. To the question "What is the basis for the distinction between intuitive and abstractive knowledge?" given that it is not the existence of the object and not a causal relation between an object and the knower, Ockham answered that they are simply different. His answer notwithstanding, it is characteristic of intuitive knowledge that it is unmediated. There is no gap between the knower and the known that might undermine certainty: "I say that the thing itself is known immediately without any medium between itself and the act by which it is seen or apprehended."

According to Ockham, there are two kinds of intuitive knowledge: natural and supernatural. In natural intuitive knowledge, the object exists, the knower judges that the object exists, and the object causes the knowledge. In supernatural intuitive knowledge, the object does not exist, the knower judges that the object does not exist, and God is the cause of the knowledge. In neither case is knowledge a relation; it is something a person has, a property of the person.

Ockham recognized that God might cause a person to think that he has intuitive knowledge of an existent object when there in fact is no such object. But such a condition is not intuitive knowledge but a false belief. Unfortunately, in acknowledging that a person has no way to distinguish between genuine intuitive cognitions and divine counterfeits of them, Ockham has in effect lost the argument to the Skeptics.

Later medieval philosophy followed a fairly straight path to Skepticism. John of Mirecourt was condemned in 1347 for holding among other things that there is no certainty of external reality because God could cause illusions to seem real. Nicholas of Autrecourt was also condemned in the same year for holding that only purely sensory reports of human experience are certain and that the only certain principle is that of contradiction, namely, that a thing cannot be and not be something at the same time. He denied that humans know that causal relations exist or that there are substances, two of many errors he credited to Aristotle, about whom he said, "In all his natural philosophy and metaphysics, Aristotle had hardly reached two evidently certain conclusions, perhaps not even a single one. . . ." The link between Skepticism and criticism of Aristotle was fairly strong, and Petrarch, in On My Ignorance and That of Many Others (1367), cited Aristotle as "the most famous" of those who do not have knowledge.


3.2.5 From scientific theology to secular science.

For most of the Middle Ages there was no split between theology and science (scientia). Science was knowledge that was deduced from self-evident principles, and theology received its principles from the source of all principles, God. In every way, theology was superior to the other sciences, according to Thomas Aquinas. By the 14th century the ideas of science and theology began to be separated. Roughly, theologians began to argue that human knowledge was much more narrowly circumscribed than earlier believed. They often exploited the omnipotence of God in order to undercut the arrogance and pretension of human reason. Their motive was to enhance the dignity of God at the expense of human reason, and in place of rationalism in theology, they promoted a kind of fideism.

Gregory of Rimini (c. 1300-c. 1358) exemplified the growing split between natural reason and theology. According to Gregory, theology is not a science, and theological propositions are not scientific. In the new view of Gregory, who was inspired by Ockham, science deals only with what is accessible to humans through natural means, that is, through the ordinary operations of their senses and intelligence. Theology in contrast deals with what is accessible in some supernatural way. Thus, theology is not scientific. The role of theology is to explain the meaning of the Bible and the articles of faith and to deduce conclusions from them. Since the credibility of the Bible rests upon belief in divine revelation and revelation upon the authority of God, theology lacks a rational foundation. Further, since there is neither self-evident knowledge of God nor any natural experience of him, humans can have only an abstract understanding of what he is.

Ockham and Gregory did not at all intend their views to undermine theology. For them, natural science is built on probabilities, not certainties. Since humans are fallible, their natural science is fallible, unlike theology, which is built upon propositions that have the authority of God. Unfortunately for theology, the prestige of natural science rose in the 16th century and skyrocketed in the 17th and 18th centuries; modern thinkers preferred coming to their own conclusions based upon experience and reason, even if these were only probable, to trusting the authority of anyone, even God. (This attitude has been called "the Faustian ethos," after Goethe's character Faust.) As the theologians tended to lose confidence in reason, other thinkers who had no or virtually no commitment to Aristotelian thought became the champions of reason and helped give birth to modern science.




3.3.1 Faith and reason.

Modern philosophers as a group are usually thought to be purely secular thinkers. Nothing could be further from the truth. From the early 17th century until the middle of the 18th century, all of the great philosophers incorporated substantial religious elements into their work. Descartes, in his Meditations (1641), offered two different proofs for the existence of God, and he asserted that no one who does not believe in a cogent proof for the existence of God can have knowledge in the proper sense of the term. Benedict Spinoza began his Ethics (1677) with a proof for the existence of God, after which he expatiated on its implications for understanding all reality. And George Berkeley explained the stability of the sensible world by relying upon God's constant thought of it.

Among the reasons modern philosophers are mistakenly thought to be primarily secular thinkers is that many of their epistemological principles, including some that were intended to defend religion, were later interpreted as subverting the rationality of religious belief. The role of Thomas Hobbes (1588-1679) and John Locke might be briefly considered in this connection. In contrast with the standard view of the Middle Ages that propositions of faith are rational, Hobbes argued that propositions of faith belong not to the intellect but to the will. To profess religious propositions is a matter of obeying the commands of a lawful authority. One need not even understand the meanings of the words professed: an obedient mouthing of the appropriate confession of faith is sufficient. In any case, the linguistic function of virtually every religious proposition is not cognitive in the sense of expressing something that is intended to represent a fact about the world but rather to give praise and honour to God. Further, in contrast to the medieval view, according to which theology is the highest science, theology is not a science at all since its propositions are not susceptible to rational dispute.

In An Essay Concerning Human Understanding (1689), Locke further eroded the intellectual status of religious propositions by making them subordinate to reason in several dimensions. First, reason can dictate what the possible content of a proposition allegedly revealed by God might be; in particular, no proposition of faith can be a contradiction. Consequently, if the proposition that Jesus is both fully God and fully man is contradictory, it cannot be revealed and cannot be a matter of faith. Also, no revelation can be communicated that contains an idea not based upon sense experience. Thus, St. Paul's experience of things "as eye hath not seen, nor ear heard, nor hath it entered into the heart of man to conceive," are things in which other people can have no faith. To move to another dimension in which reason takes precedence over faith, direct sense knowledge (what Locke calls "intuitive knowledge") is always more certain than any alleged revelation. Thus, a person who sees that someone is soaking wet cannot have it revealed to him that the person is at that moment dry. Rational proofs, in mathematics and science, also cannot be contradicted by divine revelation. The interior angles of a rectangle equal 360°, and no alleged revelation to the contrary is credible. In short, "Nothing that is contrary to, and inconsistent with, the clear and self-evident dictates of reason, has a right to be urged or assented to as a matter of faith. . . . "

What space, then, does faith occupy within the mansion of human beliefs? According to Locke, it shares a room with probable truths, those propositions of which reason cannot be certain. There are two types: claims about observable matters of fact and claims that go "beyond the discovery of our sense." Religious propositions belong to each category, as do empirical or scientific ones. That Caesar crossed the Rubicon and that Jesus walked on water belong to the first type of probable proposition. That heat is caused by the friction of imperceptibly small bodies and that angels exist are propositions that belong to the second category. (see also  probability)

While mixing religious claims with scientific ones might seem to secure a place for the former, in fact it did not. For Locke also held that whether something is a revelation or not "reason must judge," and more generally that "Reason must be our last judge and guide in everything." Although this maxim was intended to reconcile reason and revelation--indeed, he calls reason "natural revelation" and revelation "natural reason enlarged by a new set of discoveries communicated by God"--over the course of 200 years reason repeatedly judged that alleged revelations had no scientific or intellectual standing.

Although there is a strong religious element in modern thinkers, especially before the middle of the 18th century, the purely secular aspects of their thought predominate in the following discussion, because it is these that are of contemporary interest to epistemologists.


3.3.2 Impact of modern science on epistemology.

Nicolaus Copernicus (1473-1543), a cleric, argued in On the Revolutions of the Celestial Spheres (1543) that the Earth revolves around the Sun. His theory was epistemologically shocking for at least two reasons. First, it goes directly counter to how humans experience their relation to the Sun; it is everyone's prescientific view that the Sun revolves around the Earth. If science can overthrow such a belief, then scientific reasoning seems to lead to knowledge in a way that nonscientific reasoning cannot. Indeed, the nonscientific reasoning of everyday life may seem to be a kind of superstition. Second, his theory was shocking because it contradicts the view that is presented in several books of the Bible, most importantly the explicit account in Genesis of the structure of the cosmos, according to which Earth is at the centre of creation and the Sun hangs from a celestial ceiling that holds back the waters which once flooded the Earth. If Copernicus is right, then the Bible can no longer be taken as a reliable scientific treatise. Scientific beliefs about the world, then, must be gathered in a radically new way. (see also  Copernican Revolution, Copernican system)

Many of the discoveries of Galileo Galilei (1564-1642) had the same two shocking consequences. His telescope seemed to reveal that unaided human vision gives false or seriously incomplete information about the nature of celestial bodies. His mathematical formulations of physical phenomena seem to indicate that most sensory information may contribute nothing to knowledge. Like his contemporary, the astronomer Johannes Kepler, he distinguished between two kinds of properties. Primary qualities, such as shape, quantity, and motion, are genuine properties of things and are knowable by mathematics. Secondary qualities, namely, odour, taste, sound, colour, warmth, or coldness, exist only in human consciousness and are not part of the objects to which they are normally attributed. (see also  primary quality, secondary quality)


3.3.3 René Descartes.

Both the rise of modern science and the rediscovery of Skepticism were important influences on René Descartes (1596-1650). While he believed that humans were capable of knowledge and certainty and that modern science was developing the superstructure of knowledge, he thought that Skepticism presented a legitimate challenge that needed an answer, one that only he could provide.

The challenge of Skepticism, as Descartes saw it, is vividly portrayed in his Meditations. He considered the supposition that all of one's beliefs are false, being the delusions of an evil genius who has the power to impose beliefs on people unbeknownst to them. But Descartes claimed that it is not possible for all of one's beliefs to be false, for anyone who has false beliefs is thinking and knows that he is thinking, and if the person is thinking, then that person exists. Nonexistent things cannot think. This line of argument is summarized in Descartes's formula, "Cogito, ergo sum" ("I think; therefore, I am"). (see also  "Meditations on First Philosophy," )

Descartes distinguished two sources of knowledge: intuition and deduction. Intuition is an unmediated mental seeing or direct apprehension of something experienced. The truth of the proposition "I think" is guaranteed by the intuition one has of one's own experience of thinking. One might think that the proposition "I am" is guaranteed by deduction, as is suggested by the "ergo." In Objections and Replies (1642), however, Descartes explicitly says that the certainty of "I am" is also based upon intuition.

If one could know only that one thinks and exists, human knowledge would be depressingly narrow. So Descartes proceeded to broaden the limits of human knowledge. After showing that all human knowledge depended upon thought or reason, not sensation or imagination, he then proceeded to prove to his own satisfaction that God exists; that the criterion for knowledge is clearness and distinctness; that mind is more easily known than body; that the essence of matter is extension; and that most of his former beliefs are true.

Few of these proofs convinced many people in the form in which Descartes presented them. One major problem is what has come to be known as the Cartesian circle. In order to escape from the possibility that an evil genius is deluding him about everything he believes, Descartes proves that God exists. He then argues that clearness and distinctness is the criterion for all knowledge because God does not deceive man. But, since this criterion is arrived at only after the existence of God has been proven, he cannot appeal to this criterion when he presents his proof for the existence of God; hence he cannot know that his proof is cogent.


3.3.4 John Locke.

An Essay Concerning Human Understanding by John Locke (1632-1704) is often taken to be the first major empiricist work. Book I discusses innate ideas in order to deny that there are any; Book II discusses various genuine kinds of ideas; Book III discusses language with an emphasis on the meaning of words; and Book IV discusses knowledge and related cognitive states and processes.

Innate ideas are ideas that humans are born with. Rationalist philosophers, like Descartes and Gottfried Wilhelm Leibniz (1646-1716), thought that there have to be such ideas in order to explain the existence of some of the ideas which humans have. One argument for innate ideas is that, while the ideas of blue, dog, and large, for example, can be explained as the result of certain sense impressions, other ideas seem unable to be attributed to sensation. Numbers, for example, seem to be outside the realm of sensation. Another argument is that some principles are accepted by all human beings, as, for example, the principle that out of nothing nothing comes. Locke did not think either of these arguments had any force. He held that all ideas can be explained in terms of sensation, and he set as one of his projects the task of providing such an explanation. Instead of directly attacking the hypothesis of innate ideas, Locke's strategy was to refute it by showing that it is otiose and hence dispensable.

In Book II of the Essay Locke supposes the mind to be like a blank sheet of paper that is to be filled with writing. How does the paper come to be filled? "To this I answer, in one word," says Locke, "Experience." He divides experience into two types: observation of external objects and observation of the internal operations of the mind.

Observation of external objects is another description for sensation. Observation of the internal operations of the mind does not have its own word in ordinary language, and Locke stipulated "reflection" to designate it, because people arrive at ideas by reflecting on the operations of their own minds. Examples of reflection are perceiving, thinking, doubting, believing, reasoning, knowing, and willing.

An idea is anything that the mind "perceives in itself, or is the immediate object of perception." Qualities are the powers that objects have to cause ideas. Many words have dual senses. The word red, for example, might mean either the idea of red in the mind or the quality in a body that causes the idea of red in the mind. Some qualities are primary in the sense that all bodies have them. Solidity, extension, figure, and mobility are primary qualities. Secondary qualities are those powers that, in virtue of the primary qualities, cause the sensations of sound, colour, odour, and taste. Locke's view is that the phenomenal redness of a fire engine is not in the fire engine itself, nor is the phenomenal sweet smell of a rose in the flower itself. Rather, certain configurations of the primary qualities cause phenomena such as the appearance of red or the taste of sweetness, and in virtue of these configurations the object itself is said to have the quality of redness or sweetness. But there is no resemblance between the idea in the mind (phenomenon) and the secondary quality that causes it. Locke claims, without justification, as George Berkeley was later to argue, that there is, however, a resemblance between primary qualities and the ideas of them. (Locke distinguishes a third sort of quality, e.g., the power of fire to produce a new colour or consistency in wax or clay, but he makes nothing of it.)

Although Locke along with most distinguished modern philosophers repudiated Aristotelianism and the Scholasticism to which it gave rise, a doctrine of abstraction survives in his philosophy. Abstraction occurs when "ideas taken from particular beings become general representatives of all of the same kind." That is, to abstract is to ignore the particular circumstances of time and place and to use an idea to represent all things of a certain kind.

In Book IV Locke finally defines knowledge as "the perception of the connexion of and agreement, or disagreement and repugnancy of any of our ideas." He also distinguishes several degrees of knowledge. The first is knowledge in which the mind "perceives the agreement or disagreement of two ideas immediately by themselves, without the intervention of any other," which he calls "intuitive knowledge." His first examples are such analytic propositions as "white is not black," "a circle is not a triangle," and "three are more than two." But later he says, "The knowledge of our own being we have by intuition." Relying on the metaphor of light as Augustine and others had, Locke says of this knowledge that "the mind is presently filled with the clear light of it. It is on this intuition that depends all the certainty and evidence of all our knowledge."

The second degree of knowledge occurs when "the mind perceives the agreement or disagreement of . . . ideas, but not immediately." Some mediating idea makes it possible to see the connection between two other ideas. Proofs are things that show the mediating connections between ideas, and a clear and plain proof is a demonstration. Demonstrative knowledge is certain but not as evident as intuitive knowledge, says Locke, because it requires effort and attention to go through the steps needed to recognize the certainty of the conclusion.

A third degree of knowledge, "sensitive knowledge," approximates to what Duns Scotus and Ockham called "intuitive cognition," namely, the perception of "the particular existence of finite beings without us." Unlike medieval intuitive cognition, Locke's sensitive knowledge is less certain than his intuitive or demonstrative knowledge.

Beneath knowledge is probability, which is the appearance of agreement or disagreement of ideas with each other. Etymologically, probability is a likeliness to be true, and it guides in matters "whereof we have no certainty." Locke suggests that probability rests upon the testimony of others and, like knowledge, comes in degrees, which depend upon the likely veracity of the sources of the proposition. The highest degree of probability attaches to propositions endorsed by the general consent of all people in all ages. Locke may have in mind the virtually general consent of his contemporaries in the proposition that God exists. But he explicitly mentioned beliefs about causal relations, which are not perceived but inferred. To argue from such beliefs is called "an argument from the nature of things." The next degree of probability or assurance in probable propositions attaches to matters that hold not universally but for the most part, such as that persons prefer their own private advantage to the public good. This sort of proposition is typically derived from history. The next degree of probability or assurance attaches to claims about specific facts, for example, that a man named Julius Caesar lived a long time ago. Problems arise when testimonies conflict, as they often do, but there is no simple rule or set of rules that instructs one how to resolve such controversies.

In addition to these probabilities, all of which concern particular matters of fact, there are also probabilities about things that are not within the power of the senses. The existence, nature, and operation of angels, devils, microbes, magnets, and molecules all fall into this class. It is important to recognize that for people as scientific as Locke, who was a member of the Royal Society, all of these were part of the same class. It took many centuries to separate science from religion and superstition.


3.3.5 George Berkeley.

Locke is part of a philosophical tradition called empiricism, that is, the view that the sole or at least the major source of human knowledge is sensory experience. George Berkeley (1685-1753) was the next great adherent of empiricism. In his major work, Treatise Concerning the Principles of Human Knowledge (1710), he divides ideas into three types: Ideas that come from sense correspond to Locke's simple ideas of sensation. Ideas that come from "attending to the passions and operations of the mind" correspond to Locke's ideas of reflection. Ideas that come from compounding, dividing, or otherwise representing ideas, correspond to Locke's compound ideas. An apple, for example, is a compound of the simple ideas of colour, taste, smell, and figure associated with it.

In addition to ideas, what exists are spirits or souls or minds. By "spirit," Berkeley means "one simple, undivided, active being." Spirit exercises itself in two ways: in understanding and in willing. Understanding is spirit perceiving ideas, and will is spirit producing ideas. It is evident, says Berkeley, that no idea, including those of sensation, can exist outside of a mind. This is evident, not merely in virtue of the meaning of "idea" but what it means to exist. For a table to exist is for someone to see or feel it. To be an odour is to be smelled. To be a sound is to be heard. In short, for nonthinking beings, esse is percipi (to be is to be perceived). (see also  existence)

The question whether a tree falling in a virgin forest makes a sound is inspired by Berkeley's philosophy, though he never asked it in those terms. He did, however, consider the thrust of the objection and gave various answers to it. He sometimes says that a table in a room unperceived is a table that would be perceived if someone were there. This conditional response, however, is not sufficient. Granted that the table would exist if it were perceived, does it exist when it is not perceived? Berkeley's other answer is that, when no human is perceiving a table or other such object, God is; and it is his thinking that keeps the otherwise unperceived object in existence.

However strange his doctrine may initially sound, Berkeley claimed that he was merely describing the commonsense view of reality. To say that colours, sounds, trees, dogs, and tables are ideas is not to say that they do not really exist. It is merely to say what they are. To say that animals and pieces of furniture are ideas is not to say that they are diaphanous, gossamer, and evanescent. Opacity, density, and permanence are also ideas that partially constitute these objects.

Berkeley has a syllogistic argument for his main point: physical things, such as trees, dogs, and houses, are things perceived by sense, and things perceived by sense are ideas; therefore, physical things are ideas. If one objects that the first premise is false, Berkeley in reply would challenge the objector to point out one example of something that is not sensed. The only way to identify such an example is through some sensation, either by sight, touch, taste, or hearing. In this way, any proffered counterexample becomes an example of Berkeley's point.

If one objects that the second premise of the syllogism is false on the grounds that people sense things, not ideas, Berkeley would reply that there are no sensations without ideas and that it makes no sense to speak of some additional thing which ideas are supposed to represent or resemble. Unlike Locke, Berkeley does not believe that there is anything "behind" ideas in a world external to the mind. There could not be. If the alleged external objects, of which ideas are supposed to be representations, exist, then they are themselves either ideas or not. If they are ideas, then Berkeley's point that everything perceived is an idea is vindicated. If they are not ideas, then they are unperceived; in particular, they would be invisible colours, intangible textured things, odourless smells, and silent sounds. If someone objects that he can imagine trees or books in a closet unperceived, Berkeley would reply that this proves nothing except that there are imagined trees and books. People who think that there are unperceived objects are deceived because they do not take into account their own thinking of the allegedly unperceived object.

A consequence of this argument is that Locke's distinction between primary and secondary qualities is spurious. Extension, figure, motion, rest, and solidity are as much ideas as green, loud, and bitter are; there is nothing special about the former kinds of ideas. Furthermore, matter, as philosophers conceive it, does not exist and indeed is contradictory. For matter is supposedly unsensed extension, figure, and motion, but since extension, figure, and motion are ideas, they must be sensed.

Berkeley's doctrine that things unperceived by human beings continue to exist in the thought of God was also not novel. It was part of the traditional belief of Christian philosophers from Augustine onward through Aquinas and at least to Descartes that God not only creates all things but keeps them in existence by thinking of them. In this view if he were ever to stop thinking of a creature, it would immediately be annihilated.

On another matter, the doctrine of abstraction, Berkeley made a clean break with the past. Berkeley rejected it completely, because he thought it led to belief in unperceived, nonspiritual substances. Abstractionism, according to Berkeley, illicitly warrants the separation of existence from being perceived. For him every idea is particular and of a particular object. There cannot be an idea of motion in general but only of a certain body moving slowly or quickly. To reject abstract ideas is not to reject general ideas. An idea is general in virtue of "being made to represent or stand for all the other particular ideas of the same sort." That is, each general idea is a particular idea that stands for many things. (see also  universal)


3.3.6 David Hume.

Although Berkeley rejected the Lockean notions of primary and secondary qualities and matter, he retained Locke's beliefs in the existence of mind, substance, and cause as a power or secret force. David Hume (1711-76), in addition to rejecting all the Lockean notions that Berkeley did, also rejected what Berkeley had retained. His justification for this step was empiricist and scientific, for he thought that all science is empiricist and that there is no empirical justification for belief in mind or spirit.

Hume aspired to be the Newton of philosophy. As stated in A Treatise of Human Nature (1730-40), he wanted to formulate universal principles to explain "all effects from the simplest and fewest causes," but a boundary condition on these principles is that they "cannot go beyond experience." Further, the ultimate principles that humans can form will themselves lack justification. They will explain experience without having an explanation of their own. Kinds of perceptions.

Hume has a twofold division of perceptions: impressions and ideas. Impressions are perceptions that enter with "most force and violence." Ideas are "faint images" of impressions. Hume thinks the distinction so obvious that he demurs from explaining it at any length. Impressions are felt; ideas are thought, he indicates in his summary explication. He also concedes that, although one can always discern the difference between an impression and an idea by its force, sleep, fever, and madness sometimes produce ideas that approximate to the force of impressions, and certain impressions approach the weakness of ideas. But such occasions are rare.

The distinction has a problem that Hume did not notice. The impression (experience) of anger has an unmistakable quality and intensity, but it is not the case that the idea of anger always makes a person feel angry. Thinking of anger no more guarantees being angry than thinking of the idea of happiness guarantees being happy, even if thinking happy thoughts tends to make people happy. So there is a difference between the experience of anger and the idea of anger that Hume's philosophy does not capture.

In addition to impressions and ideas, perceptions can be divided into the categories of simple and complex. Whereas simple perceptions are not subject to further separation or distinction, complex perceptions are. For example, apples, although unitary objects in one view, are in fact complex perceptions; they are divisible into a certain shape, colour, texture, and aroma. It is noteworthy that for every simple impression there is a simple idea that corresponds to it and differs from it only in force and vivacity, and vice versa. So, corresponding to the impression of red is the idea of red. This does not hold true in general for complex perceptions. Although there is a correspondence between the impression of an apple and the idea of an apple, there is not always a correspondence between impressions and ideas. There is no impression that corresponds to the idea of Pegasus or a unicorn; these complex ideas do not have a correlate in reality. There are also complex impressions that do not have a corresponding idea. A traveler who has seen an extensive part of Rome nonetheless does not have an idea of Rome that corresponds in every respect to his perceptions.

Because of their correspondence, there seems to be a special connection between simple impressions and simple ideas: the former cause the latter. Hume deduces this on the following grounds. A simple impression always precedes the corresponding idea, and the idea invariably follows the conjoined impression. Thus, because of the temporal priority of impressions and the constant conjunction of impressions and ideas, Hume concludes that impressions cause ideas.

There are two kinds of impressions: sensation and reflection. Sensation "arises in the soul originally from unknown causes." Hume says little more about sensation because discussion of it belongs to anatomists and scientists. (Many late 20th-century philosophers do not accept this division between philosophy and anatomy.) To explain reflection is rather complicated because it derives from a complex mental operation. After people feel heat or cold, thirst or hunger, pleasure or pain, they form ideas of heat or cold, thirst or hunger, pleasure or pain. And, following the formation of these ideas--at a third stage of cogitation--they form from the ideas the second kind of impressions: impressions of "desire and aversion, hope and fear." These impressions are the result of reflecting on ideas caused by sensation.

Since imagination can divide and assemble disparate ideas as it will, some explanation is needed for why the mind seems to run in predictable channels. Hume says that the mind is guided by three principles: resemblance, contiguity, and cause and effect. Thus a person who thinks of one idea is likely to think of another idea that resembles it. For example, a person's thought, if one accepts Hume's account, will run from red to pink to white, or from dog to wolf to coyote. Hume also uses the principle of resemblance to explain how general ideas function. Hume agrees with Berkeley in denying that there are abstract ideas, and he affirms that all ideas are particular. Some of them, however, are used to represent many objects by inclining the mind to think of other ideas that resemble the first. These particular ideas that represent many things are general ideas. Concerning contiguity, people are inclined to think of things that are next to each other in space and time. Finally and most importantly, people associate ideas on the basis of cause and effect relations. Fire and smoke, parent and child, disease and death are tied in the mind because of their causal relations. But cause and effect relations play a more central role in Hume's thought than these brief remarks might suggest. Cause and effect.

Although people gain much information from their impressions, most matters of fact depend upon reasoning about causes and effects, even though people do not directly experience causal relations. What, then, are causal relations? According to Hume they have three components: contiguity of time and place, temporal priority of the cause, and constant conjunction. In order for x to be the cause of y, x and y must exist adjacent to each other in space and time, x must precede y, and x and y must invariably exist together. There is nothing more to the idea of causality than this; in particular, people do not experience and do not know of any power, energy, or secret force that causes possess and that they transfer to the effect. Still, all judgments about causes and their effects are based upon experience. To cite examples from An Enquiry Concerning Human Understanding (1748), since there is nothing in the experience of seeing a fire close by which logically requires that one will feel heat, and since there is nothing in the experience of seeing one rolling billiard ball contact another that logically requires the second one to begin moving, why does one expect heat to be felt and the second ball to roll? The explanation is custom. In previous experiences, the feeling of heat has regularly accompanied the sight of fire, and the motion of one billiard ball has accompanied the motion of another. Thus the mind becomes accustomed to certain expectations. "All inferences from experience, therefore, are effects of custom, not of reasoning." Thus it is that custom, not reason, is the great guide of life. In short, the idea of cause and effect is neither a relation of ideas nor a matter of fact. Although it is not a perception and not rationally justified, it is crucial to human survival and a central aspect of human cognition. Substance.

One of the cornerstones of philosophy from Plato to Berkeley was the notion of substance, that which exists in itself and does not depend upon anything else for its existence. Substance is contrasted with accident or modes of being, which exist in substances and depend on them for their existence. A dog is a substance, and its colour, shape, weight, and bark exist in the dog and depend on it for their existence. One of the reasons for Hume's place in the history of philosophy is that he denied the existence of substance, using the epistemological principles he shared, not simply with empiricists like Locke and Berkeley, but with Aristotle and Aquinas as well. As argued in the Treatise, since all human knowledge must be traced back to sensation, the idea of substance must be also. But what sensation can give rise to the idea of substance? It is not a colour, shape, sound, or taste. Substance, by its proponents' own definition, is not an accident or mode. Hume concludes, "We have therefore no idea of substance, distinct from that of a collection of particular qualities, nor have we any other meaning when we either talk or reason concerning it." What then are the things that earlier philosophers designated substances? They are "nothing but a collection of simple ideas, that are united by the imagination, and have a particular name assigned to them." Gold, to take Hume's example, is nothing but the collection of the ideas of yellow, malleable, fusible, and so on. Even the mind is only a collection, "a heap or collection of different perceptions united together by certain relations and suppos'd tho' falsely, to be endow'd with a perfect simplicity or identity." Relations of ideas and matters of fact.

Human thought concerns two kinds of things: relations of ideas and matters of fact. Relations of ideas can either be intuited, that is, seen directly, or deduced from other propositions. That a is identical with a, that b resembles c, and that d is larger than e are examples of propositions that are intuited. The opposites of true propositions expressing relations of ideas are contradictory. Arithmetic and algebra are the subjects about which there can be the most certainty. In his Treatise Hume says that geometry is almost as certain as these, but not quite, because its original principles derive from sensation, and about sensation there can never be absolute certainty. He revised his views about geometry later, and in the Enquiry he puts geometry on an equal footing with the other mathematical sciences.

In contrast with relations of ideas, matters of fact are derived from experience. Experience, however, would be quite limited if it did not include causal relations, which go beyond what is experienced. Skepticism.

Hume's discussion about relations of ideas and matters of fact gives the impression that he thought that human knowledge is possible. Relations of ideas seem to be the object of knowledge, while matters of fact seem to be the object of probability. In Part II of the Treatise he denies this and argues forcefully for Skepticism.

Until the beginning of Part IV of Book I of the Treatise, there is little or no hint of Skepticism. The distinction between knowledge (of the mathematical sciences) and probability (of matters of fact) seems to presuppose that there is knowledge. But one then discovers that Skepticism undermines it all. Although the rules of science are certain and infallible, the application of those rules by humans is uncertain and fallible because humans are prone to error. It does no good for a person to try to check his chain of reasonings because the process of checking is no more immune to error than the original calculation. How can one know that the checking process was performed correctly? And, if the checking procedure seems to identify a mistake in the original calculation, how can one determine whether the error is in the original or in the seeming identification of an error? Adding a checking procedure is in one respect worse than leaving the original calculation alone. It introduces a second event, which, like the original calculation, is possibly flawed. And it is more probable that one of two possibly flawed events is flawed than either one of the two alone. "By this means," Hume says, "all knowledge degenerates into probability." Another way to see this consequence is to consider that reason is a cause of truth. But, since all causal relations are probable, not certain, all human reasoning is at best probable.

If one thinks further about the matter, the probability of knowledge diminishes and doubt increases. Each judgment of the probability of some judgment introduces further reasons for doubt and thus lowers the overall probability. The joint probability of p and q is lower than the probability of p; and the joint probability of p, q, and r is lower than the probability of p and q. Ultimately, "when I proceed still farther, to turn the scrutiny against every successive estimation I make of my faculties all the rules of logic require a diminution, and at last a total extinction of belief and evidence." If one should say, "Surely, you are kidding," Hume's answer would be a beguiling one: In a sense, "yes," for nature has so made human beings that they cannot in fact be skeptical even though the argument for Skepticism is cogent. As Hume says in his Enquiry, people conduct their lives for the most part governed by custom and nature, not reason. Skepticism is true even though there are no Skeptics, because, as in Berkeley's philosophy, the arguments for Skepticism "admit of no answer and produce no conviction."

There is another way of expressing Hume's position. If one examines the grounds that human beings have for trusting their reasoning, one will not be able to find rational grounds. Reason cannot be rationally grounded, and the ground of rationality is wholly nonrational: "belief is more properly an act of the sensitive, than of the cogitative part of our natures."

Some people have tried to make short shrift of Skepticism by pointing out that if the Skeptic recognizes his arguments to be rationally compelling, then he must recognize the sovereignty of reason and hence the falsity of Skepticism. Hume points out that the battle against Skepticism cannot be won in this way. Skepticism is a refutation of the claims of reason. As such, one assumes the truth of rationalism in order to show that it is contradictory and hence false. In other words, Hume's proof is a reductio ad absurdum argument against belief in rationality. The Skeptical argument proceeds by arguing that, if rationalism is true, then it is not rational to be rational. Since the consequent is contradictory, the assumption that rationalism is true must be false. Thus, rationalism is false.

Hume has been called "the complete Pyrrhonist," but Hume himself denied that he was one, in large part because he did not distinguish between Pyrrhonism and Academic Skepticism, both of which, according to him, advocate the suspension of belief even as one conducts one's ordinary affairs. Hume thought such a program impossible for human beings: humans are condemned to believe. Unlike the Pyrrhonist, Hume does not suspend judgment or abandon reason. He judges according to reason because it is his nature to do so even though Skeptical arguments against reason are cogent. This philosophical schizophrenia--the use and trust in reason coupled with the recognition that rationality has no rational justification--is part of what Hume calls "mitigated Skepticism." Another part of it is restricting one's investigations to topics that are within the "narrow capacity of human understanding," namely to experience and the mathematical sciences.

Given his Skepticism, one might wonder whether it could be directed against Hume's own positive doctrine. It can. At the end of Part I of his Treatise Hume says, "Can I be sure, that in leaving all establish'd opinions I am following truth; and by what criterion shall I distinguish her, even if fortune shou'd at last guide me on her foot-steps? After the most accurate and exact of my reasonings, I can give no reason why I shou'd assent to it; and feel nothing but a strong propensity to consider objects strongly in that view, under which they appear to me." Ultimately one judges according to custom and the way nature dictates one must judge. The Enquiry Concerning Human Understanding is intended to be an accurate description of how people judge, not a justification of it.


3.3.7 Immanuel Kant.

Idealism is often defined as the view that everything which exists is mental; that is, everything is either a mind or depends for its existence upon a mind, as do ideas and thinking. Immanuel Kant (1724-1804) was not strictly an idealist according to this definition, although he called himself a "transcendental idealist." On his view, humans can know only what is presented to their senses or what is contributed by their own mind. Every sensory experience is a mixture of a sensory content, which is simply given to a person, and a spatial and temporal form, which is contributed by the mind itself. Further, if one formulates a sensory experience into a judgment, then the mind also contributes certain additional objective features: the judgment incorporates ideas of something being a substance or quality of that substance, ideas of one thing causing another, or one thing being related by necessity or by accident to another. In short, the raw data of sensory input is only a small part of what constitutes human knowledge. Most of it is contributed by the human mind itself; and, so far as human knowledge is concerned, rather than the mind trying to accommodate itself to the external world, the world conforms to the requirements of human sensibility and rationality. Kant compared his radical reorientation of the way philosophers ought to study human knowledge to the Copernican revolution in astronomy. Just as the Earth revolves around the Sun, contrary to common sense, objects conform themselves to the human mind, contrary to common sense. (see also  transcendental idealism)

Kant's idealism notwithstanding, he also believed that a world existed independent of the human mind and completely unknowable by it. This world consists of things-in-themselves, which do not exist in space and time, are not organized in causal relations, and so on, because these are elements contributed by the human mind as conditions for knowing. Because of his commitment to realism (minimal though it may be) Kant was disturbed by Berkeley's uncompromising idealism, which amounted to a denial of the external world. Kant found this incredible and rejected "the absurd conclusion that there can be appearance without anything that appears." (see also  thing-in-itself)

Kant's goal, as developed in Critique of Pure Reason (1781), was to supplant Berkeley's crude idealism with a transcendental idealism. The difference, as Kant saw it, is that, while Berkeley began empirically by noting that everything that humans are rationally justified in asserting to exist is related to consciousness, he went on to ask what necessary conditions underlie any empirical experience at all. Kant did not deny that there is empirical experience, but he was critical of Berkeley for not excavating its rational underpinnings. Kant is called a "rationalist" because he thought that the conditions for empirical experience can only be reasoned to, not discovered in, experience; he called his idealism "transcendental" because the conditions he was looking for are common to--they transcend--any experience. In his notorious "proof of an external world," he claimed that he experienced himself as an object in time, that time requires something permanent outside of his consciousness as a precondition for his existence in time, and hence that an external world exists. In other words, the claim is that inner experience presupposes an outer or external world. But few philosophers have claimed to understand why this should be so, and the very contrast of inner and outer seems to beg the question.

Kant believed that all objects of sensation must be experienced within the limits of space or time. Thus, all physical objects have a spatiotemporal location. Because space and time are the backdrop for all sensations, he called them pure forms of sensibility. In addition to these forms, there are also pure forms of understanding, that is, categories or general structures of thought that the human mind contributes in order to understand physical phenomena. Thus, every empirical object is thought to have some cause, to be either a substance or part of some substance, and so on. The structure of judgments finally leads to the question of what properties the propositions that express judgments (or knowledge) have.

From a logical point of view, the propositions that express human knowledge can be divided according to two distinctions. First is the distinction between propositions that are a priori, in the sense that they are knowable prior to experience, and those that are a posteriori, in the sense that they are knowable only after experience. Second is the distinction between propositions that are analytic, that is, those in which the predicate is included in the subject, and those that are synthetic, that is, those in which the predicate is not included in the subject. Putting the terms of these two distinctions together yields a fourfold classification of propositions. (1) Analytic a priori propositions include "All bachelors are unmarried" and "All squares have four sides." (2) Analytic a posteriori propositions do not exist, according to Kant, because, if the predicate is conceptually included in the subject, the appeal to experience is irrelevant and unnecessary. Also, the negation of an analytic proposition is a contradiction; but, because any experience is contingent, its opposite is logically possible and hence not contradictory. (3) Synthetic a priori propositions include "Every event has a cause" and "7 + 5 = 12." Although it is not part of the concept of an event that it be a cause, it is universally true and necessary that every event has a cause. And, because 12 is a different concept from seven, five, and plus, it does not include any of them singly or jointly as a part of it. (4) Finally, synthetic a posteriori propositions include, "The cat is on the mat" and "It is raining." They are straightforwardly and uncontroversially empirical propositions that are not necessary and are discoverable through observation. (see also  a priori knowledge, synthetic proposition, analytic proposition )

Kant's view that human experience is bounded by space and time and that it is intelligible only as a system of completely determined causal relations existing between events in the world and not between the world and anything outside of it has the consequence that there can be no knowledge of God, freedom, or human immortality. Each of these ideas exceeds the bounds of empirical experience and hence is banished from the realm of reason. As he said, he "found it necessary to deny knowledge, in order to make room for faith."


3.3.8 G.F.W. Hegel.

G.F.W. Hegel (1770-1831) developed his epistemology pari passu with ontology. Since his positive views are difficult and replete with technical terms, his epistemology is not susceptible of summary here. Some of his criticisms of earlier epistemological views, however, should be mentioned since they helped to bring modern philosophy to a close.

Empiricism takes cognition of particular sensed objects as the foundation for knowledge. But, Hegel argues, no sensation is purely particular. For every sensation consists of something that has a certain feature, quality, or feel, and this feature, quality, or feel is something common to other sensations and hence not particular. Also, all knowledge must be expressible in language, and all fully articulated language uses predicates, which express concepts. Even if the empiricist attempts to represent his knowledge with a single, purely demonstrative word, say, "this" or "now," his view is contradictory. For "this" is common to any indicated object, and "now" can be used to refer to any time. An analogous argument holds against anyone who, like Descartes or Kant, wants to begin with the referent of "I."

Another mistake common to empiricism and rationalism is to think that knowledge requires a correspondence between a person's beliefs and reality. The search for such correspondence is logically absurd since every such search ends with some belief about whether the correspondence holds or not, and thus one has not advanced beyond belief. Kant's distinction between the thing-in-itself and the phenomenon of consciousness is an instance of this absurdity. To make the distinction is to have the object in itself in consciousness and hence not in itself. Thus, Hegel concludes that knowledge and reality cannot be two things but must be identical. Knowledge cannot be perspectival or relative to each person; it is as absolute and objective as reality.



Contemporary philosophy begins in the late 19th and early 20th century. Much of what sets contemporary philosophy off from modern philosophy is its explicit criticism of the modern tradition and sometimes its apparent indifference to it. There are two basic strains of contemporary philosophy: Continental philosophy, which designates the philosophical style of western European philosophers, and Anglo-American, or analytic, philosophy, which includes the work of many European philosophers who immigrated to Britain, the United States, and Australia shortly before World War II.


3.4.1 Continental philosophy.

In epistemology, Continental philosophers during the first quarter of the 20th century were preoccupied with the problem of overcoming the apparent gap between the knower and the known. If a human being has access only to his own ideas of the world and not the world itself, how can there be knowledge at all? (see also  dualism)

The German philosopher Edmund Husserl (1859-1938) thought that the standard epistemological theories had become intrusive because philosophers were attending to repairing or complicating them rather than focusing on the phenomena of knowledge as humans experience them. To emphasize this reorientation of thinking, he adopted the slogan, "To the things themselves." Philosophers needed to recover the sense of what is given in experience itself, and this could only be accomplished through a careful description of phenomena. Thus, Husserl called his philosophy "phenomenology," which was to begin as a purely descriptive science and only later to ascend to a theoretical, or "transcendental," science.

Husserl thought that the philosophies of Descartes and Kant presupposed a gap between the aspiring knower and what is known and that the experience of the external world was thus dubious and had to be proven. These presuppositions violated Husserl's belief that philosophy, as the most fundamental science, should be free of presuppositions. Thus, he held that it is illegitimate to assume there to be any problem of knowledge or of the external world prior to an investigation of the matter without any presuppositions. Husserl's device to cut through the Gordian knot of such assumptions was to introduce an "epoche." In other words, he would bracket or refuse to consider traditional philosophical problems until after the phenomenological description had been completed. (see also  Cartesianism)

The epoche was just one of a series of so-called transcendental reductions that Husserl proposed in order to ensure that he was not presupposing anything. One of these reductions supposedly gave one access to "the transcendental ego," or "pure consciousness." Although one might expect phenomenology then to describe the experience or contents of this ego, Husserl instead aimed at "eidetic reduction," that is, the discovery of the essences of various sorts of ideas, such as redness, surface, or relation. All of these moves were part of Husserl's desire to discover the one, perfect methodology for philosophy in order to ensure absolute certainty.

Because Husserl's transcendental ego seems very much like the Cartesian mind that thinks of a world but does not have either direct access to or certainty of it, Husserl tried in Cartesianische Meditationen (1931; "Cartesian Meditations") to overcome the apparent gap, the very thing he had set out either to destroy or bypass. Because the transcendental ego seems to be the only genuinely existent consciousness, Husserl also tried to overcome the problem of solipsism.

Many of Husserl's followers, including his most famous student, Martin Heidegger (1889-1976), recognized that something had gone radically wrong with the original direction of phenomenology. According to Heidegger's diagnosis, the root of the problem was Husserl's assumption that there is an "Archimedean point" for human knowledge, to use Husserl's own phrase; but, there is no ego detached from the world and filled with ideas or representations, according to Heidegger. In Being and Time (1927) Heidegger returned to the original formulation of the phenomenological project as a return to the things themselves. Thus, all the transcendental reductions are abandoned. What he claimed to discover is that human beings are inherently world-bound. The world does not need to be derived; it is presupposed by human experience. In their prereflective experience, humans inhabit a sociocultural environment, in which the primordial kind of cognition is practical and communal, not theoretical or individual ("egoistic"). Human beings interact with the things of their everyday world (Lebenswelt) as a workman interacts with his tools; they hardly ever approach the world as a philosopher or scientist would. The theoretical knowledge of a philosopher is a derivative and specialized form of cognition, and the major mistake of epistemology from Descartes to Kant to Husserl was to take philosophical knowledge as the paradigm for all knowledge.

Heidegger's insistence that a human being is something that inhabits a world notwithstanding, he marked out human reality as ontologically special. He called this reality Dasein, the being, apart from all others, which is present to the world. Thus, like the transcendental ego, a cognitive being takes pride of place in Heidegger's philosophy.

In France the principal phenomenological proponent of the mid-century was Maurice Merleau-Ponty (1908-61). But he rejected Husserl's bracketing of the world, that is, his mistake in not recognizing that human experience of the world is primary, a view capsulized in Merleau-Ponty's phrase "the primacy of perception." He furthermore held that dualistic analyses of knowledge, such as the Cartesian mind-body dualism, are inadequate. In fact, no conceptualization of the world can be complete in his view. Because human cognitive experience requires a body and the body a position in space, human experience is necessarily perspectival and thus incomplete. Although humans experience material beings as multidimensional objects, part of the object always exceeds the cognitive grasp of the person just because of his limited perspective. In Phenomenology of Perception (1945), Merleau-Ponty develops these ideas (along with a detailed attack on the sense-datum theory, discussed below).

The epistemological views of Jean-Paul Sartre (1905-80) share some features with Merleau-Ponty's. Both reject Husserl's transcendental reductions, and both think of human reality as being-in-the-world. But Sartre's views have Cartesian elements that were anathema to Merleau-Ponty. Sartre distinguished between two basic kinds of being. Being-in-itself (en soi) is the inert and determinate world of nonhuman existence. Over and against it is being-for-itself (pour soi), which is the pure consciousness that defines human reality.

Later Continental philosophers attacked the entire philosophical tradition from Descartes to the 20th century for its explicit or implicit dualisms. Being/nonbeing, mind/body, knower/known, ego/world, being-in-itself/being-for-itself are all variations on a way of philosophizing that the philosophers of the last third of the 20th century have tried to undermine. The structuralist Michel Foucault (1926-84) wrote extensive historical studies, most notably The Archaeology of Knowledge (1969), in order to demonstrate that all concepts are historically conditioned and that many of the most important ones serve the political function of controlling people rather than any purely cognitive purpose. Jacques Derrida has claimed that all dualisms are value-laden but indefensible. His technique of "deconstruction" attempts to show that every philosophical dichotomy is incoherent, because whatever can be said about one term of the dichotomy can also be said of the other.

Dissatisfaction with the Cartesian philosophical tradition can also be found in the United States. The American pragmatist John Dewey (1859-1952) directly challenged the idea that knowledge is primarily theoretical; experience, he argued, consists of an interaction between a living being and his environment. Knowledge is not a fixed staring at something but a process of acting and being acted upon. Richard Rorty has done much to reconcile Continental and Anglo-American philosophy. He has argued that Dewey, Heidegger, and Ludwig Wittgenstein are the three greatest philosophers of the 20th century, specifically because of their attacks on the epistemological tradition of modern philosophy. (A.P.Ma.)


3.4.2 Analytic philosophy.

Analytic philosophy, the prevailing philosophy in the Anglo-American world in the 20th century, has its origins in symbolic logic on the one hand and in British empiricism on the other. Some of its important contributions have been nonepistemological in character, but in the area of epistemology its contributions have also been of the first order. Its main characteristics have been the avoidance of system building and a commitment to detailed, piecemeal analyses of specific issues. Within this tradition there have been two main approaches: a formal style, deriving from logic; and an approach emphasizing ordinary language. Among those who can be identified with the first method are Bertrand Russell, Gottlob Frege, Rudolf Carnap, Alfred Tarski, and W.V.O. Quine; and among those with the second are G.E. Moore, Gilbert Ryle, J.L. Austin, Norman Malcolm, P.F. Strawson, and Zeno Vendler. Wittgenstein can be situated in both groups, his early work belonging to the former tradition and his posthumous works, Philosophical Investigations (1953) and On Certainty (1969), to the latter.

Perhaps the most distinctive feature of analytic philosophy is its emphasis upon the role that language plays in the creation and resolution of philosophical problems. These problems, it is said, arise through the misuses, oversimplifications, and unwarranted generalizations of everyday language. Wittgenstein said in this connection: "Philosophy is a battle against the bewitchment of the intelligence by means of language." The idea that philosophical problems are in some important sense linguistic (or conceptual) is called the "linguistic turn." Commonsense philosophy, logical positivism, and naturalized epistemology.

Three of the most notable achievements of analytic philosophy are commonsense philosophy, logical positivism, and naturalized epistemology. G.E. Moore (1873-1958) made a defense of what he called the commonsense view of the world. According to Moore, virtually everybody knows certain propositions to be true, such as that the Earth exists, that it is very old, and that other persons now exist on it. Furthermore, any philosophical theory that runs counter to this commonsense view can be rejected out of hand as mistaken. All forms of idealism fall into this category. Wittgenstein, for whom certainty is that "which stands fast for all of us," extended this view. In On Certainty he argued that certitude is connected with action and that "Action lies at the bottom of the language game."

The development of logical positivism (also called logical empiricism) was a product of the Vienna Circle under the leadership of the German logical empiricist philosopher Moritz Schlick, and it became a dominant form of philosophy in England with the publication of A.J. Ayer's Language, Truth, and Logic (1936). Logical positivism holds that all significant propositions are either those of logic or mathematics on the one hand or those of science on the other. Since the utterances of traditional philosophy (especially metaphysics) fall into neither of these groups, they are unverifiable in principle and accordingly can be rejected as nonsense. The only legitimate function for philosophy is conceptual analysis, i.e., the clarification of various notions, such as "probability" or "causality."

W.V.O. Quine (b. 1908), in "Two Dogmas of Empiricism" (1950), launched an attack upon the notion that there is a difference in kind between analytic and synthetic statements. Quine argued powerfully that the so-called difference is one of degree. In a later work, Word and Object (1960), Quine developed a new type of philosophy, which he called "naturalized epistemology." He rejected the notion that epistemology has a normative function and claimed that its only legitimate role is to describe the way knowledge is actually obtained. In effect, its function is to describe how present science arrives at the beliefs accepted by the scientific community. Perception and knowledge.

To a great extent the epistemological interests of analytic philosophers in the 20th century have been concentrated upon the relationship between knowledge and perception. The major figures in this development have been Bertrand Russell, G.E. Moore, H.H. Price, C.D. Broad, A.J. Ayer, and H.P. Grice. Although their views differed considerably--Russell, Broad, and Ayer were phenomenalists, Grice was a defender of the causal theory of perception, and Moore attempted to construct a theory of direct realism--all of them were defenders of sense-data theory (see below).

Sense-data theory was criticized by proponents of the so-called theory of appearing, such as G.A. Paul and W.H.F. Barnes, who claimed that the arguments for the existence of sense-data are spurious. Those arguments assume, for example, that because a penny looks elliptical from a certain perspective, it follows that there exists an elliptical object (sense-datum), which an observer is directly apprehending. They denied the inference, saying that the introduction of a separate entity, a sense-datum, does not follow from the fact that a circular object looks elliptical and to believe that it does is simply to misdescribe certain common perceptual situations. The most powerful attack on sense-data theory was generated by J.L. Austin in Sense and Sensibilia (1962).

Many philosophers, in turn, rejected the theory of appearing. They felt that puzzles about the status of illusions and other visual anomalies still require explanation. Their aim was to give a coherent account of how knowledge is possible despite the existence of perceptual error. Realism and phenomenalism are the two main types of theories developed to account for these difficulties.

Both realism and phenomenalism have had numerous variants. Two forms of realism, direct (naive) realism and representative realism (also called "the causal theory"), are historically important. Realism.

Realism is both a metaphysical and an epistemological theory. The realist is committed to two principles: first, that some of the objects apprehended through perception are public and, second, that some of those objects are mind-independent. It is especially the second of these notions that distinguishes realists from phenomenalists.

The realist believes that there is an intuitive commonsense distinction among various classes of entities perceived by human beings. One class consists, among others, of headaches, thoughts, pains, or desires, and the other of tables, rocks, planets, persons, animals, and certain physical phenomena such as rainbows, lightning, and shadows. The metaphysical aspect of realism sees the former as mental, the latter as physical. A realist metaphysics maintains that the classes are mutually exclusive. What a realist epistemology adds to this metaphysics is that mental entities are private, whereas physical objects are public. By "private" it is meant that each item belonging to the category of the mental is apprehensible by one person only. Thus, only one person can have a particular headache or a particular pain. In contrast, physical objects are public; more than one person can see or touch the same chair. (see also  intuition)

The realist also believes that items belonging to the class of the physical are mind-independent. What is meant by this notion is that the existence of these objects does not depend upon their being perceived by anyone. Thus, whether or not a particular table is being seen or touched by someone has no effect upon its existence. Even if nobody is looking at it, it would still exist (other things being equal). But this is not true of mental phenomena. If somebody is not actually having a headache, realists would deny that the headache exists. A headache is thus mind-dependent in a way in which tables, rocks, and shadows are not.

Realist theories of knowledge thus begin by assuming the public-private distinction, and most realists start by assuming that one does not have to prove the existence of mental phenomena. These are things of which each person is directly aware, and there is no special "problem" about their existence. But they do not assume this to be true of physical phenomena. As the existence of visual aberrations, illusions, and other anomalies shows, one cannot be sure that in any perceptual situation one is apprehending physical objects. All a person can be sure of is that he is aware of something, an appearance of some sort, say of a bent stick in water; but whether that appearance corresponds to anything actually existing in the external world is an open question.

In the Foundations of Empirical Knowledge (1940) Ayer called this difficulty "the egocentric predicament." When a person looks at what he thinks is a physical object, such as a chair, what he is directly apprehending is a certain visual appearance. But such an appearance seems to be private to that person; it seems to be something mental and not publicly accessible. What then justifies the individual's belief in the existence of supposedly external objects--i.e., physical entities that exist external to the human mind? Direct realism and representative realism are the two main theoretical responses to this challenge.

Both direct realism and representative realism rely strongly on sense-data theory. The technical term "sense-datum," which played an important role in the development of versions of both theories, is sometimes explained by using examples. If one is hallucinating and sees pink rats, one is seeing a sense-datum. Although no real rats are there, one is having a certain visual sensation as of coloured rats, and this sensation is what is called a sense-datum. The image one sees with one's eyes closed after looking fixedly at a bright light is another example. But, even in normal vision, one can be said to be apprehending sense-data. For instance, in looking at a round penny from a certain angle, one will see the penny to be elliptical. In such a case, there is an elliptical sense-datum in one's visual field. This last example was held by Broad, Price, and Moore to be particularly important, for it makes a strong case for holding that one always sees sense-data, whether perception is normal or abnormal.

According to defenders of sense-data theory, what these examples have in common is that in every perceptual act one is directly aware of something. A sense-datum is thus frequently defined as an entity that is the object of direct perception. By "direct" these philosophers mean that no inference is necessary in order to apprehend these entities. According to Broad, Price, and Ayer, sense-data differ from physical objects in having the properties they appear to have; i.e., they cannot appear to have properties they do not really have. The problem for a realist who accepts sense-data is to show how these private sensations allow justification of the intuitive belief that there are physical objects which exist outside of the individual's perception. Russell in particular tried to show in such works as The Problems of Philosophy (1912) and Our Knowledge of the External World (1914) how knowledge of the external world could be built up from such mental, private apprehensions.

During the 20th century direct realism took many forms; indeed there were direct realists, such as James J. Gibson who, in The Ecological Approach to Visual Perception (1979), rejected sense-data theory and claimed that the outside aspects (the physical surfaces) of physical objects are normally directly observed. But many realists, such as G.E. Moore and his followers, believed that the existence of sense-data must be accepted. Moore took the unusual step of suggesting that such sense-data might not be mental entities but could be a physical part of the surface of the perceived material object. Thompson Clarke in "Perceiving Physical Objects and Surfaces" (1965) went beyond Moore in arguing that one normally directly perceives the whole physical object itself.

All of these views have problems in dealing with perceptual anomalies. In fact, Moore, in his last published paper, "Visual Sense-Data" (1957), abandoned the attempt to defend direct realism. He held that, because the elliptical sense-datum one perceives when one looks at a round coin cannot be identical with the circular surface of the coin, one cannot be seeing the coin directly but only the sense-datum. Hence, one cannot have direct knowledge of external objects.

Because of the problems associated with direct realism, many philosophers, including H.H. Price, H.P. Grice, and Robert E. French, have argued for the causal theory, that is, the theory of representative realism. This is an old view whose most famous exponent in early modern philosophy was Locke. It is also sometimes called "the scientific theory" because it seems to be supported by findings in optics and physics. According to this form of realism there are real physical objects that exist external to the human mind, and there are also sense-data (or their equivalents, such as so-called mental representations). Visual perception is then explained as follows. Light is reflected from external objects, moves through space according to well-known laws of physics, is picked up by the human visual system, which includes the eye, the optic nerve, and the retina, and then is ultimately processed by the brain. This is a causal sequence. Light causes a reaction in the eye, that reaction is the cause of a response in the optic nerve, and so forth. The last event in this causal sequence is "seeing." (see also  science, vision)

What one is apprehending in such a case is a mental representation (sense-datum) of the original object; and, through various processes in the brain, this representation gives human beings a depiction of the object as it is. Visual illusion is explained in various ways, but usually as the result of some anomaly in the causal chain that gives rise to distortions and other types of aberrant visual phenomena. In such a view, human observers are directly aware of mental representations, or sense-data, and only indirectly aware of the physical objects that cause these data in the brain.

The difficulty with this view is that, since one cannot compare the sense-datum that is directly perceived with the original object, one cannot ever be sure that it gives an accurate representation of it; and therefore human beings cannot know that the real world corresponds to their perception of it. They are still confined within the circle of appearance after all. It thus seems that neither version of realism satisfactorily solves the problem it began with. Phenomenalism.

In light of these difficulties with realist theories of perception some philosophers, so-called phenomenalists, proposed a completely different way of analyzing the relationship between perception and knowledge. In particular, they rejected the distinction between independently existing physical objects and mind-dependent sense-data that direct realism presupposes. They claimed that either the very notion of an independent existence is nonsense because human beings have no evidence for it or that what is meant by "independent existence" must be reinterpreted in such a way as not to go beyond the sort of perceptual evidence human beings do or could have for the existence of things. In effect, these philosophers challenged the cogency of the intuitive ideas that the ordinary person supposedly has about independent existence.

All variants of phenomenalism are strongly verificationist in thrust. That is, they wish to maintain that belief in an external world must be capable of verification or confirmation, and this entails that such a belief cannot be acceptable if it goes beyond the realm of possible perceptual experience.

Phenomenalists have thus tried to analyze in wholly perceptual terms what it means to say that any object, say a tomato, exists. They claim that any such analysis must start by deciding what is meant by a tomato. In their view a tomato is something that has certain properties, including a certain size, weight, colour, and shape. If one were to abstract the total set of such observed properties from the object, nothing would be left over; there would be no presumed Lockean "substratum" that supports these properties and which is itself unperceived. There is thus no evidence in favour of such an unperceivable feature, and no reference to it is needed in explaining what a tomato or any so-called physical object is.

To talk about any existent object is thus to talk about a collection of perceivable features localized in a particular portion of space-time. Hence, what one means by a tomato is something that in principle must be perceivable. Accordingly, to say that a tomato exists is either to describe a collection of properties that an observer is actually perceiving or a collection that such an observer would perceive under certain specified conditions. To say, for instance, that a tomato exists in the next room is to say that, if one went to that room, one would see a familiar reddish shape, would obtain a certain taste if one bit into it, or would feel something soft and smooth if one touched it. To speak about that tomato's existing unperceived in the next room thus does not entail that it is unperceivable. In principle, everything that exists is perceivable. Therefore, the notion of existing independently of perception has been misunderstood or mischaracterized by both philosophers and nonphilosophers. Once it is understood that objects are merely sets of properties and that such collections of properties are in principle always perceivable, the notion that there is some sort of unbridgeable gap between people's perceptual evidence and the existence of an object is just a mistake, a confusion between the concepts of actually being perceived and of being perceivable.

In this view, perceptual error is explained in terms of coherence and predictability. To say with truth that one is perceiving a tomato means that one's present set of perceptual experiences and an unspecified set of future experiences will "cohere." That is, if the object a person is looking at is a tomato, then he can expect that, if he touches, tastes, and smells it, he will receive a recognizable grouping of sensations. If the object he has in his visual field is hallucinatory, then there will be a lack of coherence between what he touches, tastes, and smells. He might see a red shape but not be able to touch or taste it. (see also  coherence theory of truth, sense)

The theory is generalized to include what others would touch, see, and hear as well, so that what the realists call "public" will also be defined in terms of the coherence of perceptions. A so-called physical object is public if the perceptions of many persons cohere or agree, and otherwise it is not. This explains why a headache is not a public object. In similar fashion, a so-called physical object will be said to have an independent existence if the expectations of future perceptual experiences are borne out. If tomorrow, or the day after, a person has similar perceptual experiences to those he had today, then he can say that the object he is perceiving has an independent existence. The phenomenalist thus attempts to account for all the facts that the realist wishes to explain without positing the existence of anything that transcends possible experience.

The criticisms of this view tend to be technical. Generally speaking, however, realists have objected to it on the ground that it is counterintuitive to think of a tomato as being a set of actual or possible perceptual experiences. The realist argues that human beings do have such experiences, or under certain circumstances would have them, because there is an object out there that exists independently of them and is their source. Phenomenalism, they contend, has the implication that, if no perceivers existed, then the world would contain no objects; and, if this is a consequence of the view, then it is surely inconsistent both with what ordinary persons believe and with the known scientific fact that all sorts of objects existed in the universe long before there were any perceivers. But its supporters deny that phenomenalism carries such an implication, and the debate about its merits remains unresolved.



In the late 1970s a series of developments occurred in a variety of intellectual fields that promise to cast new light on the nature of the human mind. There have been explosive advances in neuroscience, psychology, cognitive science, neurobiology, artificial intelligence, and computer studies. These have resulted in a new understanding of how seeing works, how the mind forms representations of the external world, how information is stored and retrieved, and the ways in which calculations, decision procedures, and other intellectual processes resemble and differ from the operations of sophisticated computers, especially those capable of parallel processing.

The implications for epistemology of these developments are equally exciting. They promise to give philosophers new understandings of the relationship between common sense and theorizing, that is, whether some form of materialism which eliminates reference to mental phenomena is true or whether the mental-physical dualism which common sense assumes is irreducible, and they also open new avenues for dealing with the classical problem of other minds. It is too early to make an assessment of the relevance for epistemology of what has already been achieved in these areas. There is no doubt, however, that these advances are revolutionary and that a new area of intellectual discovery has begun. ( Av.S.)

인식론 (認識論, epistemology). (epistemology '지식', '참된 앎'이라는 뜻의 그리스어 epistm에서 유래)

인간의 인식의 기원·본질·한계 등을 연구하는 철학의 한 분야. 

"인간은 어떻게 알 수 있는가", "안다는 것은 무엇인가", "우리는 무슨 권리로 '안다'고 말할 수 있는가" 등의 문제를 탐구하는 학문이다 ( 물질). 인간의 탐구대상이 인간이라면 인식론은 그 탐구의 중요한 부분이 될 것이다. 앎이란 인간 삶의 중요한 부분을 구성하는 것이기 때문이다. 현명하게 살아가기 위해서는 바깥 세계를 알고 이해해야 한다. 오늘날의 문명과 과학의 발달은 바로 인간의 세계이해를 반영하는 것이다. 눈부신 과학의 성취로 때로 인간은 세계에 관한 진리를 손에 쥔 것 같은 느낌을 갖기도 하지만, 인간은 또한 언제나 오류를 범할 수 있는 가능성을 안고 있으며, 이는 과학의 역사가 잘 말해주고 있다. 따라서 인간은 자신을 둘러싸고 있는 세계와 그 세계에 대한 이해를 가능하게 하는 인간 자신의 인식 능력을 탐구해야 할 필요에 직면하게 된다. 곧 인식론은 지식에 관한 지식을 목표로 하고 있다.

인식론의 역할과 다른 분야와의 연관

우선 형이상학과의 연관성을 살펴보면 경험과학의 발달과 더불어 형이상학적 탐구의 많은 부분들은 초월적이고 비과학적이라는 이유로 포기되었고, 철학적 분석의 대상에서도 제외되었음을 알 수 있다. 그러나 최근 세계의 일반적 구조와 그 궁극적 실재의 문제를 다루는 형이상학은 그것에 대한 앎의 문제를 다루는 인식론자들에게 주요관심사로 떠오르고 있다.

지식의 근원을 탐구한다는 점에서 심리학과 인식론 사이에 구분이 없었으나 20세기에 들어와 심리학은 철학적 방법을 거부하고 경험과학으로서 자리잡았다. 그러나 여전히 두 분야는 서로에게 유용한 부분들이 많다. 인식론자의 개념분석은 인지에 대한 심리학자의 탐구에 도움이 될 수 있고 인식론자는 숫자, 운동, 외부 대상들에 대한 어린아이들의 초기 인지형성이나 인간의 지각 경험, 기억, 무의식 등에 대한 심리학적 연구에서 도움을 얻을 수 있다.

심리철학은 비교적 최근에 대략적으로나마 자신의 영역을 갖게 된 분야이다. 심리철학은 인식론의 문제보다는 심신, 자아, 정신활동의 다양한 측면(예를 들면 의지·욕구·자기기만·느낌 등)에 관한 문제들에 집중하고 있다. 이를 위해 심리철학은 인식론과 심리학에서의 연구 결과들에 익숙해 있어야 하며, 또 인식론자는 심리철학의 연구 결과에 영향을 받지 않을 수 없게 되었다.

논리학은 인식론과는 언뜻 보기에는 별 연관이 없어 보인다. 그러나 논리학자들은 논리적 법칙들의 궁극적 근거나 상호 배타적 논리체계들의 양립 가능성 문제 등에 직면해 인식론적인 고려를 하지 않을 수 없으며 인식론자는 인간을 규정짓는 이성적·논리적 사유의 본성을 설명해야 한다. 또한 인식론적 분석에 논리학은 유용한 도구가 되며, '인식논리'와 같은 영역에서는 인간의 인지현상을 논리적으로 형식화하려는 시도도 이루어지고 있다.

현대인식론의 쟁점

'안다'·'앎'·'지식'이란 말들의 모든 용법을 포괄하는 하나의 정의를 내리기란 불가능하다. 인식론자들이 가장 관심을 갖는 것은 "무엇이 그러함을 안다"고 말하는 경우, 즉 명제적 지식에 관해서이다. 이러한 명제적 지식은 "……을 할 줄 안다", "(누구)를 안다", "왜 ……인지를 안다", "(장소)를 안다"와 같은 지식과 구분된다. 인식론자들이 명제적 지식에 관심을 갖는 이유는 명제가 진리치를 갖기 때문이다.

감각적 지각

감각자료 이론

세계는 시공 속에 일정 기간 지속적으로 존재하는 물질적 사물의 세계로 생각된다. 이 사물들은 상호작용하며 인간의 신체에도 영향을 미친다. 인간은 감각기관을 통해 그러한 영향을 받아들이며 결국 그것으로써 물질적 외부세계를 의식하게 된다. 인간이 언제나 감관을 통해 사물을 본다면 인간이 참으로 보는 것은 실제의 사물이 아니라 감관에 주어져 있는 감각자료라는 생각으로 유도될 수 있다. 이렇게 감각자료로서의 대상과 실제 대상을 구분하는 경우 문제는 어떻게 둘이 연결되며, 감각자료를 보는 것으로부터 어떻게 실제 대상에 관한 지식을 이끌어내는가이다. 몇몇 감각자료 이론가들은 인과적 지각이론에 의존해 답하고자 한다. 즉 감각자료는 외부의 물리적 대상들에 의해 촉발되어 존재한다는 것이다. 그러나 이 경우 물리적 대상이 존재하는지 직접적으로 알 수 없으며, 감각자료가 물리적 대상이 아닌 다른 것에 의해 촉발되었을 가능성을 배제할 수 없다는 약점을 갖는다. 이러한 난점에 대처하는 한 방법은 데이비드 흄과 존 스튜어트 밀, 20세기 중엽의 경험주의자들에 의해 시도되었던 현상론의 방법이다.


현상론자들은 물리적 대상에 대해 의미있게 말하기 위해서는 현상, 즉 나타나 보이는 것에 대한 확신만으로 족하다고 본다. 예를 들어 갈색이고, 네모지며, 만지면 매끄럽고, 누르면 딱딱하다는 등의 경험으로 책상에 대해(책상이 이 모든 현상과 독립적으로 존재함을 우선적으로 보이지 않고도) 충분히 이야기할 수 있다는 것이다. 이러한 현상론에 대해서는 다음과 같은 비판이 있어왔다. ① 현상론은 이미 물리적 대상을 전제로 하며, ② 대상이 나타나 보이는 방식이 무한할 수 있으므로 감각경험을 기술하는 환원문이 무한할 수 있으며, ③ 대상의 존재를 표시하는 문장은 정언적인 진술문인 데 반해 그것과 같은 의미를 갖는다고 하는 환원문은 가언적인 문장이다. 그러나 어떤 정언문도 가언문으로 의미 손실 없이 환원될 수 없다. 전후 현상론의 발전과 그 실패는 이 문제에 대한 급격한 전환이 필요함을 느끼게 했다. 관념론적 해결방식은 많은 철학자들에게 외부세계의 독립된 존재에 대한 인간의 확신과 어긋나는 것으로 보였다. 그래서 어떤 인식론자는 잠정적으로 이 확신 자체에 대한 탐구로부터 문제를 접근했다.

보증의 차원

합리주의적 전통에서는 외부세계에 대한 확실성을 이성 능력 속에서 구하려 했다. 이성은 절대적 진리, 필연적 진리를 직관적·직접적 방식으로 우리에게 제공한다고 생각되었다. 그러나 단번에 사물의 본질을 꿰뚫는 이러한 지적 직관 능력은 인간 지식에서 언제나 보이는 오류 가능성 때문에 많은 사람들에게 의심을 샀다. 오류 가능성을 받아들인다면 합리주의적 이상은 회의주의에 빠지게 될 것이다. 회의주의자는 확실한 지식의 가능성을 부정하고 개연적 믿음의 가능성만을 받아들인다. 그러나 오류 불가능한 지식의 가능성을 부정한다고 해서 확실한 지식의 가능성을 부정할 필요는 없을 것이다. 믿는다는 것은 무엇이 어떠한 경우임을 받아들이는 것이다. 이것은 개연적 앎의 한 종류이다. 개연적 지식에는 개연성의 정도가 개입되며 이 개연성의 정도를 결정하는 증거가 중요한 문제로 부각된다. 증거는 감각적 지각이나 기억, 그러한 것들로부터의 추론, 다른 사람과의 교류를 통해 얻어지는 정보들이다. 증거가 적합한가 아닌가의 문제는 더욱 복잡하다. 이는 적합성을 결정할 기준의 문제 때문이다. 증거의 적합성 여부는 언제나 상대적으로만 결정될 수 있고, 상식적·합리적인 사람의 의견에 비추어 결정될 수밖에 없다. 이렇게 적절한 증거를 확보한 믿음은 정당화된 믿음이며 확실한 지식일 수는 있지만 절대적 진리에 대한 지식과는 구분된다.


현대인식론이 전통인식론과 궤를 달리하게 되는 분기점은 언어적 문제에 대한 깊은 관심이다. 이 관심은 인간의 사유가 언어로 이루어지고 있다는 문제의식에서 생겨난 것이다. 전통인식론자들이 마음 안에서 일어나는 인지과정에 관심을 기울인 반면 현대인식론자들은 언어의 구조를 살핌으로써 인간의 세계에 대한 사유구조를, 그리고 나아가 언어와 세계와의 관계를 밝혀보려 한다. 귀납은 연역과 함께 논리적 추론의 한 유형으로 구분된다. 개별적 경우들에 대한 관찰로부터 보편 명제로의 이행으로 설명되는 귀납은 개연적 결론만을 유도하는 자연과학의 방법으로 주장된다. 그뿐만 아니라 귀납은 대략적 일반화나 잠정적 가설 설정을 해야 하는 일상사의 방법이기도 하다. 그러나 관찰된 경우(예를 들면 '모든 관찰된 백조는 희다')로부터 관찰되지 않은 경우(예를 들면 '모든 백조는 희다')로의 이행을 허용하는 귀납이 과연 정당한 논리적 추론의 방식일 수 있는가에 관한 논란은 그치지 않고 있다. 합리주의의 핵심은 인간의 이성 능력이 다른 어떤 것에서도 얻을 수 없는 정보의 근원이라는 주장이다. 이성은 존재·통일성·실체·원인 따위의 근본 개념들과 모순율(한 진술과 그것의 부정이 동시에 참일 수 없다는 원리)과 같은 원리에 관한 지식을 제공한다는 것이다. 그리고 이렇게 순수한 선천적 지식은 오류불가능한 것으로서 전통적으로 받아들여졌다. 그러나 문제는 오직 이성만이 알 수 있는 진리가 있는가 하는 것이다. 논리학자들이 인간의 개념체계의 근거를 이루는 원리들을 재구성하거나 수정한다는 사실은 이성이 인간의 마음에 절대적 진리를 심어놓았다는 합리주의의 주장을 약화시키는 것이다.

합리주의적 방향

다음의 인식 이론들은 '이성, 감각경험, 개념과 언어' 중 무엇을 강조하느냐에 따라서 대별된 것이다.

칸트 이전의 합리주의

고대 그리스의 합리주의는 플라톤의 형상이론에 명확히 표현되어 있다. 그리스 철학자들은 세계를 합리적인 것으로 간주하고 그 법칙과 영원한 구조, 질서를 이해하려 했다. 피타고라스 학파(BC 6세기 번성)는 그것을 '수'(數)로 보려 했으며, 파르메니데스는 '부동(不動)의 일자(一者)', 아낙사고라스는 누스(nous:이성 혹은 지력)로 이해하려 했다. 형상(form, idea)에 관한 가장 두드러진 묘사는 플라톤의 〈향연〉에서 발견된다. 많은 아름다운 대상들을 보고 그에 대한 반성을 한 사람은 어느 순간 갑자기 최고의 아름다움에 대한 일별을 갖게 되는데, 이 아름다움은 영원하고, 생성하지도 소멸하지도 않으며, 시간·장소·사람·사물에 대해 상대적인 것이 아니라고 한다. 아름다움의 형상과 더불어 선(善)과 정의의 형상들이 〈국가론〉에서 논의되고 있다. 형상론은 이후 인식론에 합리주의적 토대를 제공하며 존재론에는 영원한 불변의 실체를, 그리고 도덕에는 절대적 기준을 제공하게 된다.

아리스토텔레스는 플라톤의 합리주의를 완화시키기는 하지만 거부하지는 않았다. 아리스토텔레스는 그의 〈윤리학〉·〈형이상학〉에서 플라톤의 형상개념을 공격했다. 그는 형상이 개별적 사물과 독립해서 존재하는 것이 아니라 개별자들 속에 보편적 형식으로 존재한다고 보았다. 아리스토텔레스는 초월적 형상에 관한 지식이 진리획득에 본질적이라는 이론을 거부함에도 불구하고, 궁극적 진리는 추론적인 앎의 과정에서 지적 직관으로 비약함으로써 얻어질 수 있다고 생각했다. 지적 직관은 이전에 주어진 것들로부터의 발전이 아니라 전적으로 그 자체가 초월적인 그 무엇이며 영원하고 오류불가능한 것이다. 이것이 능동적 이성이며, 이것에 의해 인간은 모순율과 같은 근본 원리들을 알게 된다. 스토아 학파, 에피쿠로스 학파, 그리스 회의주의자들의 현존하는 단편적 글 속에서도 합리주의적 요소를 발견할 수 있다. 특히 제논·에피쿠로스·크리시포스·포세이도니우스·카르니아데스·에네시데무스의 글 속에 그 요소는 현저하게 드러나고 있다.

교부철학자들은 추론을 중시했으며 연역적 논의와 세밀한 분석에 치중했다. 11, 12세기에는 아리스토텔레스의 〈영혼에 관하여 De anima〉에서 능동적 이성과 수동적 이성에 관한 논의에 상당한 관심이 기울어졌다. 13세기의 토마스 아퀴나스는 누스를 독립된 실체로 간주한 플로티누스 및 그 이후의 아라비아 철학자들의 견해를 거부하고 감각경험과 지성의 호혜적 관계를 인정했다. 그러나 지식이 이성적 능력을 소유한 근거 위에서만 가능하다고 봄으로써 합리주의자로서의 면모를 강하게 보여주고 있다.

르네 데카르트는 현대 합리주의의 아버지로 불린다. 그의 합리주의는 〈방법서설 Discours de la méthode〉(1637)에서 제시된 방법과 명석판명한 지식에 관한 설명에서 잘 드러내고 있다. 그는 신과 인간과 자연에 대한 확실한 지식을 구축하기 위해 이제껏 참으로 받아들였던 모든 것을 의심에 붙이는 방법론적 회의를 제창한다. 그의 인식론 전체에서 근본적으로 중요성을 지닌 또다른 규칙은 명석판명한 관념을 지식의 기준으로 삼은 것이었다. 이것은 주의깊은 마음에 명백하게 드러나는 것으로서 일종의 지적 직관과 같은 것이다. 이에 의해 마음은 절대적으로 확실한(오류불가능한) 진리를 획득하게 된다. 후에 데카르트는 진리에 대한 마지막 보장을 신에게서 구함으로써 명석판명한 지적 직관의 절대성과 궁극성에 제한을 두는 것처럼 보인다. 그러나 신이 존재한다는 사실 또한 직관에 의해 드러난다고 함으로써 그의 논의는 악순환에 빠질 위험에 놓이게 된다.

데카르트와 마찬가지로 스피노자 또한 수학의 방법과 같은 엄밀한 방법을 강조했다. 이러한 태도는 그의 명저 〈기하학적 질서에 따라 증명된 윤리학 Ethica in Ordine Geometrico Demonstrata〉(1675 완성)의 제목에서도 드러나고 있다. 그에 의하면 모든 참된 지식은 수학적 필연성을 지니는 것으로 새로운 결론은 단계마다 매번 논리적 필연성에 따라 증명되어야 하는 것이다. 우리에게 알려지는 세계는 합리적·필연적인 하나의 체계이며, 그것은 유일한 것이어야 한다. 스피노자는 그것을 신 또는 실체(substance)라고 말한다. 데카르트와 스피노자처럼 라이프니츠도 수학적 방법을 철학에 적용하고자 했다. 그러나 그들과는 달리 라이프니츠는 이 세계는 무수히 많은 단자(monade)들로 구성되어 있으며 각 단자는 자신의 성질을 갖고 있는 실체라고 생각했다. 각각의 단자는 다 다르며 상호 연관이 없다('드나들 수 있는 창문이 없다'). 따라서 그것이 가진 지식은 선천적이며 내부에서 오는 것이 된다. 그러나 영원하고 필연적 진리를 아는 단자들(즉 인간의 영혼)이 있는데 이는 반성적이고 관조적인 의식에 의해 가능하게 된다. 이때의 지식은 명석 판명하며 이성적 진리의 형태를 갖게 된다.

칸트의 비판적 합리주의

비판철학을 정초한 18세기의 칸트는 합리주의자였지만 조금 달랐다. 그의 주저 〈순수이성비판 Kritik der reinen Vernunft〉(1781)은 합리주의에 대한 많은 비판을 담고 있다. 칸트가 대륙의 세 합리주의자들과 달랐던 점은 크게 3가지로 요약될 수가 있다. ① 철학적 방법은 수학적 방법과는 다르다. ② 감각적 경험이 사물에 대한 현상적 지식을 제공한다. ③ 감각적 경험을 넘어서는 영역에 대한 지식은 가능하지 않으며, 감성적 조건에 제한을 받지 않는 순수이성의 활동은 세계에 관한 우리의 지식을 확장해주지 못한다. 즉 신의 존재 또는 비존재는 지식의 대상이 될 수 없다. 이러한 상이점에도 불구하고 칸트의 언어는 라이프니츠-볼프 학파에 속했고 그의 사유 또한 합리주의적 경향을 강하게 나타내고 있다. 대상이 우리의 경험에 주어지기 이전의 인식능력들의 선천적 형식과 원리들을 강조하고, 그것들이 지니는 필연성·보편성이 우리의 경험을 객관적으로 토대짓는 역할을 강조하고 있다는 점에서 그렇다. 인식능력의 형식이나 범주개념, 원리들은 경험을 가능하게 하고, 규정지으나 그들 자체는 결코 경험적으로 주어지지 않는다. 독단적 합리주의에서는 모든 선천적 지식은 개념적이고 분석적이라고 본 반면 칸트는 그것을 종합적이라고 보았으며, 따라서 그것은 우리에게 특수한 종류의 확장적 지식을 제공한다고 보았다. 선천적·종합적 지식은 철학적 지식의 참된 유형으로 간주된다. 그러나 선천적·종합적 지식은 인간의 대상에 대한 지식의 한계를 정하는 것으로서, 물자체의 차원과는 아무 연관이 없다. 우리는 대상을 현상으로서 알 뿐 우리의 경험과 독립된 대상 그 자체로서 아는 것이 아니라는 것이 선험적 관념론의 주된 명제이다.

칸트 이후의 합리주의

절대적 관념론은 헤겔에 의해 최초로 명확한 형태로 제시되었다. 〈피히테와 셸링 철학 체계의 차이 Differenz des Fichte'schen und Schelling'schen System der Philosophie〉(1801)는 그의 관념론의 근원을 밝혀주고 있다. 헤겔에 있어 근본적 실재는 정신으로 이루어져 있으며 물질은 그것의 한 현현이다. 절대정신은 정·반·합의 변증법적 과정을 통해 자기실현을 향해 나아간다. 변증법은 논변이나 사유의 방식만을 가리키는 것이 아니고 그 자체가 세계의 운동 또는 발전의 논리를 구성한다. 절대적 관념론은 19세기 중엽 독일의 대학들에서 맹위를 떨쳤다. 이러한 상황은 위대한 사상가를 배출하지는 못했지만 포이어바흐와 같이 헤겔 철학에 강력한 자연주의적 전환을 시도함으로써 마르크스 철학의 기초를 다져놓기도 했다. 20세기초 영국에서는 브래들리, 미국에서는 로이스, 그리고 약간 늦게 이탈리아에서는 크로체에 의해 절대적 관념론이 꽃피었다. 이외에 비록 영국 관념론의 몰락을 가져오긴 했지만 G. E. 무어의 소박한 실재론을 합리주의 진영에 포함시킬 수 있고, 당시의 관념론이나 실재론과는 전혀 다른 철학을 정초한 현상학의 후설이나, 미국의 블샤드도 합리주의 철학자라고 할 수 있다.

경험주의로의 방향

고대 그리스와 중세의 경험주의

고대 그리스의 초기 철학자들의 관심은 자연세계의 본질에 관한 것이었다. 도대체 무엇이 존재하는지, 존재의 기본단위는 무엇인지 알고자 했으며, 그것이 물이나 공기, 불이 아닐까 생각했다. 이들의 탐구방법은 관찰과 관찰된 것에 대한 사변이었기 때문에 때로 이들은 경험주의자라고도 불린다. 원자론을 처음 시도한 로이키포스는 파르메니데스의 일원론에 반(反)하여 복수원자론을 내세웠으며, 실재를 이루는 입자들 사이에는 빈 공간이 있다고 주장했다. 복수원자론은 최고의 자연철학자 데모크리토스가 지지한 이론이다.

인간의 감각적 지각을 다룬 가장 중요한 고대 철학 작품은 의심할 바 없이 플라톤의 〈테아이테토스〉이다. 이 대화편에서 플라톤은 "감각이 곧 지식이다", "올바른 판단이 지식이다", "판단을 가능하게 하는 이성이 지식이다"라는 가정을 검토하면서 이들 모두를 부정하고 있다. 수학자 테아이테토스는 감각이 곧 지식이라고 주장했다. 관찰자에게 드러나는 그대로가 사물이라는 생각은 프로타고라스를 비롯한 소피스트들의 것이며 소크라테스도 이에 동정적이었다. 아리스토텔레스는 플라톤과는 달리 감각 경험의 중요성을 강조하고 있다. 그는 "감각적 지각 없이는 배움도 이해도 있을 수 없다"고 했다. 왜냐하면 자연에 관하여 추론해낼 아무것도 없을 것이기 때문이다. 아리스토텔레스에게 감각이 제공한다고 하는 지식은 단순한 감각과 인지적 파악 모두를 포함하는 것으로서, '흰색 대상을 디아레스의 아들로 파악하는 것'과 같은 복합적인 지각을 가리킨다. 감각적 지각에서 취할 수 있는 최상의 것은 감각적 지각이 합리적으로 논쟁할 수 있는 일반화를 가능하게 한다는 것이다. 그러나 아리스토텔레스는 그러한 일반화가 자연세계에 적용되는 한, 또한 언제나 감각 경험에 의해 시험되어야 한다고 덧붙였다. 다른 경험주의자들과 마찬가지로 아리스토텔레스 역시 세계에 관한 사유는 감각이 제공하는 증거에 준해서 참·거짓의 여부가 밝혀져야 한다는 분명한 태도를 지녔다. 아리스토텔레스 이후의 가장 중요한 경험주의는 스토아 학파와 에피쿠로스 학파에 의해 전개되었다.

중세 철학자들

13세기 알베르투스 마그누스 때까지 그당시 철학의 유일한 목표는 합리적 논의를 통해 그리스도교적 믿음을 정당화하는 것이었다. 따라서 당연히 자연과학은 정체되었다. 알베르투스는 대상에 관한 지식 획득에서 감각적 관찰이 지니는 역할을 강조했으며, 그의 가르침은 가장 뛰어난 제자였던 토마스 아퀴나스에게 이어졌다. 아퀴나스는 때로 비판적이긴 하나 아리스토텔레스의 경험주의 성향을 많이 받아들이고 있다. 그러나 신기하게도 자연과학은 이들 도미니쿠스 계통의 아리스토텔레스 추종자들이 아니라 이들에 대한 비판세력이었던 옥스퍼드를 중심으로 한 프란키스쿠스 계통의 플라톤주의자들 덕분에 발전했다. 이들은 우선 수학자들이었고 자연과학의 발달은 수학을 감각적 관찰의 결과에 적용할 때 가능하다고 믿었다. 이들 중 특히 영향력 있던 인물은 로저 베이컨이었다. 자연과학을 신학의 굴레에서 벗어나게 하려는 14세기의 움직임은 형이상학 분야에서 이성의 적합성을 의심한 회의주의에 의해 더욱 촉진되었다. 지성에 의해 알려지는 보편적 개념이 지식의 참된 대상이라는 생각은 도전을 받게 되고, 참된 대상은 개별적이고 구체적인 것이라고 생각되었다. 둔스 스코투스가 명명한 구체적 '개별성'(thisness)이 무시할 수 없는 개념으로 자리잡았다.오컴은 감각적 직관을 통한 개별적 존재에 관한 앎을 지식의 근본으로 간주했다.

근대와 현대의 경험주의

초기 선구자들

17세기말 프랜시스 베이컨은 후기 저서 〈신 오르가논 Novum Organum〉(1620)에서 새로운 과학을 위한 방법론을 제시했다. 그것은 경험적 방법으로, 개별적 관찰에서 일반화로 나아가는 귀납법이다. 데카르트와 동시대인으로서 에피쿠로스 철학의 부활을 꾀한 가상디는 프랑스뿐만 아니라 영국에서도 그 영향력을 발휘했다. 그는 "감각에 주어지지 않는 것은 지성에 주어지지 않는다", "감각에는 오류가 없다"는 에피쿠로스의 주장을 되풀이함으로써 감각주의자라는 비난을 받기도 했다.

고전적 영국의 경험주의자들

〈인간 오성론 An Essay Concerning Human Understanding〉 서두에서 인간의 마음은 백지와 같은 것으로서 타고난 능력은 갖고 있지만 생득적 관념은 갖고 있지 않다고 주장했다. 아무것도 씌어 있지 않은 마음이라는 칠판 위에 감각을 통해서 관념들이 씌어지게 된다는 것이다. 그러나 관념을 갖는 것이 곧 아는 것인가? 여기서 마음과 대상, 관념, 지각의 3가지를 구별할 필요가 있다. 로크의 시대에는 마음은 실제로 존재하는 실체로 이해되었으며 물질적 실체와는 구별되어 쓰였다. 로크는 마음에 관해 당시의 일반적인 이해에 따랐다. 예를 들어 마음은 책상과 같은 대상을 직접 알지는 못하며 색깔이나 부드러움 따위의 관념들을 통해서 알게 된다. 이것이 이른바 '지각의 베일'을 통해 대상에 관한 지식을 얻게 된다고 하는 로크의 표상이론이다. 즉 대상을 지각하는 것이 아니라 대상에 의해 주어지는 관념을 지각한다는 것이다.

로크의 경험론은 당대의 여러 비판자들 중 특히 버클리에 의해 난점들이 부각되었다. 버클리가 로크의 표상이론을 극복한 방법은 물질세계의 존재를 부인함으로써였다. 버클리는 관념이 물질세계를 표상하는 것이라면 우리는 결코 물질세계에 대한 지식을 가질 수 없다고 생각했다. 우리가 아는 것은 오로지 관념뿐이며, 그것이 나타내는 대상 세계 자체의 차원에 우리는 결코 도달할 수 없기 때문이다. 따라서 경험주의의 입장을 고수하면서 지식의 가능성을 설명하는 길이란 존재하는 것, 실재하는 것은 관념을 소유하는 정신뿐임을 주장했다. 관념들은 마음에 의해 지각됨으로써 존재한다. 즉 "존재한다는 것(esse)은 지각된다는 것(percipi)이다". 버클리는 존재는 주관적·개별적 정신 안의 관념에 의존하는 것이 아니라 이들 관념을 창조해내는 어떤 보편적 정신에 의존하는 것이라고 함으로써, 주관주의나 상대주의의 문제를 피하고 있다. 은 관념이 물리적 대상과 동일시될 수 있다는 버클리의 생각을 거부했다. 로크의 표상이론은 외부세계를 있는 그대로 알 수는 없다는 회의주의적 사고를 표현하고 있는데, 흄이 로크의 이러한 이론을 발전시켰다.

지식의 가장 기초적인 단위는 감각적 인상(impression)으로서, 그것은 강렬함과 생생함으로 특징지어진다. 관념은 인상의 퇴색한 이미지로, 만일 우리가 지금 푸른색을 보고 있다면 지금 우리에게 주어진 것은 인상이며, 후에 이 색을 기억할 경우 그때 푸른색은 관념이 된다. 인상과 관념은 각각 단순하거나 복합적일 수 있다. 인간의 모든 감각 경험들은 궁극적으로 단순한 인상들로 환원되어 설명될 수 있다. 흄에게 있어 실재하는 것은 감각된 것이며 우리 마음에 존재하는 것은 오직 지각들뿐이므로, 이 한계를 벗어나는 것에 있어서도 지식은 불가능하게 된다. 따라서 단순한 감각 인상으로 환원될 수 없는 것은 존재하지 않는다고 말할 수는 없어도 알 수는 없게 된다. 이렇게 확실한 지식의 대상이 될 수 없는 것들이 있다. 합리주의자들이 대체로 그에 대한 확실한 앎이 가능하다고 생각했던 사물의 지속적 존재(또는 실체성), 인과성, 자아의 존재가 그러한 것들이다.

경험주의에 대한 평가와 현대적 경향들

18세기의 평가에 의하면 경험주의자들에 대한 보다 풍부한 고찰은 토머스 리드에 의해 이루어졌다. 리드는 경험주의자들이 감각(sensation)과 지각(perception)을 구분하지 않은 것을 비판하면서, 더구나 버클리와 흄은 상식과 전혀 걸맞지 않는 입장을 초래시킨다고 공격했다. 로크·버클리·흄에 있어서의 감각에 대한 강조는 도덕감 이론을 거쳐 궁극적으로는 공리주의로 이어지는 18세기 윤리학에 심대한 영향을 미쳤다. 프랑스와 이탈리아에서 프랑스의 감각주의자인 콩디야크의 영향은 지대했다. 그는 인간의 모든 경험이 감각에 기초함을 주장했다. 영국에서는 유물론자인 홉스가 이미 한 세기나 일찍 모든 의식은 감각에 근거하며 또한 감각은 물질적 작용임을 논했다. 18세기 의사였던 데이비드 하틀리와 산소 발견자인 조지프 프리스틀리를 비롯하여 프랑스의 여러 유물론자들은 콩디야크의 감각주의를 지지했고, 그들의 주장은 사실상 생리학이나 심리학에 가까운 것이었다. 감각주의와 함께 19세기 실증주의 또한 경험주의의 한 분파였다. 실증주의 학파의 창시자 콩트는 〈실증 철학 강의 Cours de philosophie positive〉(1830~42)에서 실증주의의 원리들을 정초했다. 그는 과학과 기술의 발전에 주목하여 새로운 과학인 사회학에 자연과학의 방법을 적용해야 한다고 주장했다. 영국에서는 밀이 경험주의와 실증주의를 수용했다. 독일어권에서는 19세기말 실증주의 학파가 생겨났는데 이들은 이전의 실증주의의 단순성에 대해 비판적 태도를 취했다. 이 학파에는 '실재에 관한 (유물론적) 철학'의 저자 칼 뒤링과 '경험비판론'의 창시자 리하르트 아베나리우스가 있었으나 가장 두드러졌던 사람은 빈대학의 물리학자인 에른스트 마흐였다. 20세기초 순수한 논리적 기초를 중시하는 새로운 실증주의가 탄생하게 되었다. 이들은 수리논리의 선구자인 프레게·러셀·화이트헤드에 힘입은 바가 크다. 이들 중에는 슐리크·카르나프·바이스만·노이라스 등이 있다. 이들은 모든 의미있는 언명은 경험에 의해 검증 또는 반증되거나 아니면 논리적 명제라고 함으로써, 형이상학이나 신학적 명제들을 의미있는 언명의 영역 밖으로 몰아냈다.

개념적 사유


인식론에서 합리주의적 입장과 경험주의적 입장은 그 강조점이 다르긴 하나, 사유작용은 모두 인정하고 있다. 사유는 이성도 아니고 감각 경험도 아닌 것으로서, 전체 인지 경험의 부분을 이루고 있다. 앎으로 이어지기도 하고 앎을 확장시키기도 하는 사유는 개념을 매개로 하여 진행된다. 이때 개념은 경험주의자들이 말하는 이른바 인상이나 관념과는 구분되어야 하며, 또 사유의 대상으로 간주되어서도 안 된다. 개념은 단순한 감각이나 기억보다 차원 높은 정신활동의 단계에 들어오는 것으로서, 언어적인 것을 포함하는 지성적인 그 무엇으로 여겨진다. 개념에 관한 한 가지 입장은 개념이 마음속에 존재하는 내적인 대상이라고 하는 것이다. 로크는 사유의 대상인 개념(로크에게는 '일반관념')은 이름(일반단어)을 갖는다고 생각했다. 개념을 정신적 대상으로 보는 견해에서는 물질적 존재들의 세계 이외에, 심리적 사건들의 세계와 또 정신의 대상으로 존재하는 개념들의 세계가 있게 된다. 또다른 입장은 개념을 실체가 아닌 능력으로 간주하는 것이다. 추상적 관념을 마음속에 잠재된 한 성향(disposition)이나 경향(propensity)으로 보았던 흄에게서 유래하는 이 견해에 따르면 개념을 갖는다는 것은 어떤 성향 혹은 능력을 갖는 것이 된다.

개념이 능력이라는 입장은 그것이 실체라는 입장보다 희망적으로 보이지만 문제가 없는 것은 아니다. 먼저 성향은 직접 탐구될 수가 없고 오직 성향되어진 것의 결과만을 볼 수 있기 때문에 모호함을 피할 수 없다. 또 정신은 개념을 만들어내기도 하는데, 만일 개념이 정신적 능력이라면 어떻게 정신이 스스로 능력을 만들어낼 수 있는지가 문제될 것이다.


인식론자들은 언어 문제에 깊은 관심을 보여왔다. 특히 그들은 사유 속에서의 언어 사용에 관심을 갖는다. 그러나 그들은 주로 논리적 사유에 관심을 가졌고 언어를 논리적 법칙에 따르는 순전히 논리적인 도구로 생각하는 경향이 있었다. 모든 사유 또는 개념 활동이 언어적인 것은 아니다. 사유 안에 사용되는 도구로써 언어가 어떻게 사유에 영향을 미치는지는 문제로 남아 있다. 그러나 언어와 사유를 명확하게 분리해낼 수도 없다. 인간이 보다 반성적으로 되어갈수록 사유의 도구로써 언어 사용이 그 사유에 점점 더 많은 영향을 미치게 되고 마침내 사유와 언어 사이의 구분 자체가 힘들어진다. 사유는 여러 언어적 수단에 의해 수행될 수 있지만, 그런 개념적 사유가 과연 지식을 확장시킬 수 있는지는 인식론의 문제로서 남아 있다. 이 문제는 인식론적 문제뿐만 아니라 보편자의 문제와 같은 형이상학적 문제들을 포함하므로 언어에 관한 연구만으로는 해결될 성질의 것이 아니다.


일반명사로 표현되는 보편자의 존재는 여러 세기에 걸쳐 철학자들 사이에서 논쟁을 일으켰다. 보편자 이론은 복잡한 문제를 해결하기 위해 제시되었다는 사실이 상황을 복잡하게 만드는 요인이 되고 있다. 소크라테스와 플라톤은 형상론 혹은 이데아론을 발전시켰다. 이 이론에 따르면, 예컨대 아름다운 사물들은 아름다움 자체와 구분되며 오직 아름다움(보편자)을 알 때에야 비로소 실재에 대한 지식을 얻을 수 있다고 보았다. 아리스토텔레스는 형상이 독립된 존재를 갖는다는 것을 비판하고, 보편자를 사물의 일반적 성질(예컨대 붉음이나 단단함 같은) 속에 두었다. 보편자 논쟁은 3세기 신플라톤주의의 창시자 중 하나인 폴피리, 5세기초 로마의 보이티우스를 거쳐 11, 12세기에 뜨겁게 달아올랐고, 13세기에는 아리스토텔레스의 입장이 많이 받아들여졌다. 14세기 들어 보편자는 사유 혹은 개념 작용 속에 추상으로, 심지어는 이름으로만 존재한다는 견해들이 등장했다. 이를 유명론(唯名論 nominalism)이라고 하는데, 현대의 관점에서 보면 유명론("보편자는 단어 혹은 단어들의 집합일 뿐이다")보다는 개념론("보편자는 개념이다")에 가깝다.

근대에 이르러 경험주의자들은 보편자를 경험에서 추상개념으로 간주했고 합리주의자들은 어떤 보편자들은 선험적으로(a priori) 알려진다고 주장했다. 현대에는 아리스토텔레스적인 실재론과 개념론이 많이 받아들여졌지만, 실증주의자들은 유명론을 제창했다. 한편 관념론자들과 수리논리주의 학파의 일원들은 플라톤적 실재론을 내세웠다. 인식론에서 보편자 문제의 해결을 위해서는 ① 일반명사가 사유 안에서 사용되는 방식에 대한 이해, ② 속성과 그것이 속하는 사물 사이의 연관이 어떠한 것인지를 규명해줄 속성에 관한 탐구, ③ 속성들의 동일성 혹은 유사성에 대한 설명, ④ 개념 활동 안에 내재하는 성향적 요인들에 대한 규정, ⑤ '개념을 갖는다'는 것은 무엇인가에 대한 해답을 필요로 한다. 보편자의 존재를 지지하는 일은 실재 존재로나 분류 원리로나 어려운 일이 아닌 듯하다. 인식론자에게 중요한 문제는 그렇다면 인간 사유에 존재론적 적합성을 확립하는 이론에 대한 필연적인 근거를 그러한 보편자 이론이 제공할 수 있는가 하는 것이다.


이상의 논의는 서양철학의 입장에서 제한적이고 개략적으로 인식론을 살펴본 것인데 여기서 다룬 것 외에도 도덕적 의식이나 미적 가치 평가에 대한 논의 등에서도 인식론적 문제들을 추출해낼 수 있다.

Macropaedia 글


4 Bibliography

BIBLIOGRAPHY. The texts of the classics mentioned below for which specific editions have not been noted are available in many English-language translations; two notable collections are The Loeb Classical Library and Oxford Classical Text series.


4.1 The history of epistemology:

(Ancient):An excellent collection on skepticism is MILES BURNYEAT (ed.), The Skeptical Tradition (1983). For Greek Skepticism in particular, see CHARLOTTE L. STOUGH, Greek Skepticism: A Study in Epistemology (1969). The chief epistemological works of PLATO are his Meno, Theaetetus, and Republic, especially Books V-VII. The views of ARISTOTLE can be found in On the Soul, Metaphysics, Book IV, ch. 5 and 6, and Posterior Analytics, Book I, ch. 3. The locus classicus for ancient skepticism is R.G. BURY (trans.), Sextus Empiricus, 4 vol. (1933-49), in The Loeb Classical Library series. From among the voluminous writings of AUGUSTINE, see Against the Academicians, trans. by MARY PATRICIA GARVEY (1942, reissued 1978).

(Medieval): For the period as a whole, see appropriate articles in The Cambridge History of Later Greek and Early Medieval Philosophy, ed. by A.H. ARMSTRONG (1967); and The Cambridge History of Later Medieval Philosophy: From the Rediscovery of Aristotle to the Disintegration of Scholasticism, 1100-1600, ed. by NORMAN KRETZMANN, ANTHONY KENNY, and JAN PINBORG (1982). For the thoughts of ANSELM OF CANTERBURY, see his Proslogium, ch. 1, and On Truth. THOMAS AQUINAS, Summa Theologiae, discusses the soul in general in part I, question 77, and the intellectual powers of the soul in part I, question 79. The views of JOHN DUNS SCOTUS can be found in the relevant sections of his Philosophical Writings, trans. by ALLAN WOLTER (1962, reprinted 1987); and in A.P. MARTINICH, "Duns Scotus on the Possibility of an Infinite Being," Philosophical Topics, supplementary vol. 80, pp. 23-29 (1982). The ideas of WILLIAM OF OCKHAM can be found in the relevant sections of his Philosophical Writings, trans. by PHILOTHEUS BOEHNER (1957, reissued 1967).

(Modern): Two excellent and now classic histories of early modern philosophy from different perspectives are EDWIN ARTHUR BURTT, The Metaphysical Foundations of Modern Physical Science, rev. ed. (1972), which emphasizes the effect of modern science on philosophy; and RICHARD H. POPKIN, The History of Scepticism from Erasmus to Spinoza, rev. and expanded ed. (1979), which emphasizes the rediscovery of skepticism in the 16th century. RENÉ DESCARTES's greatest work is Meditations on First Philosophy, trans. by JOHN COTTINGHAM (1986; originally published in Latin, 1641). JOHN LOCKE attacks the doctrine of innate ideas in Book I of his An Essay Concerning Human Understanding, ed. by PETER H. NIDDITCH (1975), while his position on knowledge is developed in Books II and IV. A good introduction to Locke's thought is JOHN W. YOLTON, Locke: An Introduction (1985). The best work of GEORGE BERKELEY is A Treatise Concerning the Principles of Human Knowledge, ed. by KENNETH WINKLER (1982); a more popular presentation of his views is Three Dialogues Between Hylas and Philonous, ed. by ROBERT MERRIHEW ADAMS (1979). DANIEL E. FLAGE, Berkeley's Doctrine of Notions: A Reconstruction Based on His Theory of Meaning (1987), discusses a central but neglected aspect of Berkeley's epistemology. DAVID HUME's most expansive discussion of knowledge is in Book I of A Treatise of Human Nature, 2nd ed., edited by L.A. SELBY-BIGGE and rev. by P.H. NIDDITCH (1978). A later and more accessible statement of Hume's view is presented in An Enquiry Concerning Human Understanding, ed. by ERIC STEINBERG (1977). IMMANUEL KANT, Critique of Pure Reason, trans. by NORMAN KEMP SMITH (1929, reissued 1978; originally published in German, 1781), is Kant's greatest work. The best clear, brief, and accurate explanation of Kant's epistemology is A.C. EWING, A Short Commentary on Kant's "Critique of Pure Reason" (1938, reprinted 1987). An important book that rejects the view of Kant as a phenomenalist or subjective idealist is HENRY E. ALLISON, Kant's Transcendental Idealism: An Interpretation and Defense (1983). For G.F.W. HEGEL's criticisms of Kant, see his Lectures on the History of Philosophy, trans. from German by ELIZABETH S. HALDANE and FRANCES H. SIMSON (1896, reprinted 1974), part III, section iii, B. A major study on the relationship between Kant and Hegel is ROBERT B. PIPPIN, Hegel's Idealism: The Satisfactions of Self-Consciousness (1989).

(Contemporary): A short and readable history of Continental philosophy is ROBERT C. SOLOMON, Continental Philosophy Since 1750: The Rise and Fall of the Self (1988). MARTIN HEIDEGGER, Being and Time, trans. by JOHN MACQUARRIE and EDWARD ROBINSON (1962, reissued 1978; originally published in German, 1927), presents an alternative epistemological scheme. JOHN DEWEY, The Quest for Certainty: A Study of the Relation of Knowledge and Action (1929, reissued 1979), is an attack on modern epistemology by an American pragmatist. RICHARD RORTY, Philosophy and the Mirror of Nature (1979), a history of modern and contemporary philosophy, has attracted great attention as an attack on classical epistemology from an analytically trained philosopher. PAUL FEYERABEND, Against Method, 2nd ed. (1988), advocates what he describes as "an anarchistic theory of knowledge." KARL R. POPPER, Objective Knowledge: An Evolutionary Approach, rev. ed. (1979), argues against a tradition that goes back at least to Aristotle and rejects the subjective interpretation of knowledge, according to which knowledge is located in individual people.

The 20th-century literature on perception and knowledge is vast. A good general collection is ROBERT J. SWARTZ (ed.), Perceiving, Sensing, and Knowing (1965, reissued 1976), which includes two important attacks upon sense-data theory--W.H.F. BARNES, "The Myth of Sense-Data," and G.A. PAUL, "Is There a Problem About Sense-Data?"--and a strong defense of the sense-datum view by C.D. BROAD, "The Theory of Sensa." The most important pre-World War II books on perception and knowledge are BERTRAND RUSSELL, The Problems of Philosophy (1911, reissued 1988), and Our Knowledge of the External World (1926, reissued 1972); G.E. MOORE, Philosophical Studies (1922, reissued 1970), especially the important articles defending sense-data theory, "Some Judgments of Perception" and "The Status of Sense-Data"; H.H. PRICE, Perception (1932, reprinted 1981), which invokes the notion of a sense-datum in defense of the causal theory of perception; and ALFRED J. AYER, The Foundations of Empirical Knowledge (1940, reissued 1971), which merges sense-data theory with the principles of logical positivism. Notable works since World War II include GILBERT RYLE, The Concept of Mind (1949, reprinted 1984), which defends a sophisticated form of epistemological behaviourism; RODERICK M. CHISHOLM, Perceiving: A Philosophical Study (1957); and J.L. AUSTIN, Sense and Sensibilia (1962), which contains a withering assault on the sense-data theory from the standpoint of ordinary-language philosophy. Surfaces, perception, and knowledge are discussed in THOMPSON CLARKE, "Seeing Surfaces and Physical Objects," in MAX BLACK (ed.), Philosophy in America (1965); JAMES J. GIBSON, The Ecological Approach to Visual Perception (1979, reissued 1986); and AVRUM STROLL, Surfaces (1988). The theory of representative realism is given a sophisticated defense in FRANK JACKSON, Perception (1977); and S. ULLMAN, "Against Direct Perception," Behavioral & Brain Sciences, 3(3):373-415 (September 1980), attacks direct realism, especially J.J. Gibson's version of that theory, from a standpoint of modern cognitive science. A comprehensive survey of the literature from about 1980 to 1984 on direct realism and representative realism is to be found in EDMOND WRIGHT, "Recent Work in Perception," American Philosophical Quarterly, 21:17-30 (January 1984).

Knowledge and the commonsense view of the world are discussed by G.E. MOORE, "A Defence of Common Sense," in his Philosophical Papers (1959); LUDWIG WITTGENSTEIN, On Certainty, trans. from German (1969); NORMAN MALCOLM, Thought and Knowledge (1977); and JOHN R. SEARLE, Intentionality: An Essay in the Philosophy of Mind (1983). An excellent simple survey of the impact of computer studies, work in artificial intelligence, neuroscience, and neurobiology on our knowledge of other minds is found in PAUL M. CHURCHLAND, Matter and Consciousness: A Contemporary Introduction, rev. ed. (1988).

Two excellent anthologies are HAROLD MORICK (ed.), Challenges to Empiricism (1972, reprinted 1980); and PAUL K. MOSER and ARNOLD VANDER NAT (eds.), Human Knowledge: Classical and Contemporary Approaches (1987). EDMUND L. GETTIER, "Is Justified True Belief Knowledge?" in Analysis, 23:121-23 (June 1963), is considered by many to be a decisive refutation of the justified, true-belief analysis of knowledge. NOAM CHOMSKY, Language and the Problems of Knowledge (1988), discusses innateness, language, and psychology. RODERICK M. CHISHOLM, Theory of Knowledge, 3rd ed. (1989); and ROBERT AUDI, Belief, Justification, and Knowledge (1988), are two good introductions to standard epistemological problems. ( A.P.Ma./Av.S.)

  • 개론서
    • 인식론 : J. 헤센, 이강조 역, 서광사, 1987
    • 인식론 : D. W. 햄린, 이병욱 역, 서광사, 1986
    • 인식론 : W. V. 라인 외, 정대현 역, 종로서적출판, 1984
  • 고대
    • 국가론(플라톤전집 1) : 플라톤, 최민홍 역, 성창출판사, 1986
    • 파이돈(플라톤전집 6) : 플라톤, 최민홍 역, 성창출판사, 1986
  • 중세
    • 토미스트인식론 - 토미스트 실재론과 인식비판 : E. 질송, 이재룡 역, 서광사, 1993
  • 근세
    • 인성론 : D. , 이준호 역, 서광사, 1994
    • 방법서설 : R. 데카르트, 권오석 역, 홍신문화사, 1989
    • 인간지성론 (세계사상대전집 22) : J. 로크 외, 이용순 역, 대양서적, 1980
  • 근대
    • 인식과 존재 - 순수이성의 이율배반과 선험적 관념론 : 문성학, 서광사, 1991
    • 순수이성비판 : I. 칸트, 최재희 역, 박영사, 1983
    • 역사철학강의 전2(삼성보급판세계사상전집 6, 7) : G. W. F. 헤겔, 김종호 역, 삼성출판사, 1982
  • 현대
    • 논리적 관점에서 : W. V. O. 콰인, 허라금 역, 서광사, 1993
    • 인식의 해석학 - 인식의 철학 Ⅰ : O. F. 볼노우, 백승균 역, 서광사, 1993
    • 진리의 양면성 - 인식의 철학 Ⅱ : O .F. 볼노우, 백승균 역, 서광사, 1993
    • 확실성의 탐구 : J. 듀이, 김준섭 역, 백록, 1992
    • 말과 행위 : J. L. 오스틴, 김영진 역, 서광사, 1992
    • 물질과 의식 : P. M. 처치랜드, 편집부 역, 서광사, 1992
    • 확실성에 관하여 : L. 비트겐슈타인, 이영철 역, 서광사, 1990
    • 지식이란 무엇인가 : 정대현, 서광사, 1990
    • 철학의 문제들 : B. 러셀, 박영태 역, 서광사, 1989
    • 마음의 문제 - 데카르트에서 비트겐슈타인까지 : N. 맬컴, 류의근 역, 서광사, 1987
    • 시간과 존재 (청하신서 13) : M. 하이데거, 문학과 사회연구회 편역, 청하, 1986



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