“Plato’s Theory of Knowledge” is a previously unpublished article from Dr. Gordon H. Clark’s papers. It is presented to the public for the first time here on the Gordon H Clark Foundation website. If you notice any typos on the typed document please email the administrator at douglasdouma@yahoo.com

**Items from the unpublished papers of Dr. Gordon H. Clark should not be considered his definitive statement on the particular topic addressed. These papers are being provided for educational value. For Dr. Clark’s official positions consult his published writings.**

Notes: This article of Dr. Gordon H. Clark’s is from post-1970 as evidenced by the works cited.  Unfortunately a center section of the document is missing.

Unpublished 5. Plato’s Theory of Knowledge (typed)


Gordon H. Clark

Knowledge and its definition have concerned philosophers for centuries, and many different theories of knowledge have been created. There are various aspects of the attempt to define knowledge, and Plato has examined several of them in his dialogues. Eventually Plato formulated his theory of Idea, believing that Ideas are the true objects of knowledge.

Before studying the world of Ideas, Plato examines in the Theaetetus the world of appearance which the senses reveal. He discusses the claim of the sense world to yield knowledge, a claim which Protagoras extended to meaning that it yields the only knowledge we can ever have. Plato refutes Protagoras’ theory that man, through his constantly changing perception, which is knowledge, is the measure of all things. Protagoras’ theory was influenced by Heraclitus’ assumption of universal flux. The Heraclitean philosophy was opposed by Parmenides’ doctrine that all motion and change is an illusion.

In the Theaetetus, which deals with the sensible world, Plato defines what he will and will not accept from Heraclitus’ doctrine which applies to the sense world, and he does the same with Parmenides’ denial of change in the Sophist where the world of unchanging reality is considered. The Theaetetus discusses the claim of the senses to yield knowledge, and it proves that if one only considers the world of appearance and ignores the world of true being, knowledge cannot be extracted from sensible objects. The dialogue leads to the conclusion that true knowledge must have different objects, not sensible things, but Ideas and truths about them. The Theaetetus is an unsuccessful attempt to define knowledge.

At the beginning of the Theaetetus, Plato asks the question with which most of the dialogue is concerned: What is knowledge? The dialogue begins with the common sense answer to the question, that knowledge comes to us externally through the senses. Plato considers this perception to be the lowest form of cognition. From this common sense view he works upward toward the world of intelligible rather than sensible objects in order to determine whether knowledge can be found at these lower levels without having to continue upward to the intelligible world.

Following this pattern, the young Theaetetus tries to answer Socrates’ question on the definition of knowledge by identifying knowledge with perception. Socrates begins a dialectical treatment of Theaetetus’ idea.

…dialectic means a co-operative inquiry carried on in conversation between two or more minds that are equally bent, not on getting the better of the argument, but on arriving at the truth.1

A hypothesis is made by one speaker, and it is corrected and improved until its complete meaning is clear. Criticism may then result in rejection of the hypothesis or in a new idea which should be closer to the truth. In this dialogue three suggestions are made and all rejected.

Plato first tries to reveal the full meaning of the assertion that perception is knowledge. Theaetetus’ identification of perception with knowledge parallels Heraclitus’ theory that all things are in motion and Protagoras’ doctrine that man is the measure of all things. Plato shows how a complete, sensualistic theory of knowledge – that everything that is perceptible changes constantly from one state of being to another – can be derived from these two theories.

Plato accepts some of the elements from these doctrines as true when they have the necessary qualifications. Later he shows what he will not accept from the theories. Plato agrees with Protagoras that perception is infallible and that what appears to a person in perception is true for him, although Plato and Protagoras may have interpreted this assumption differently. Heraclitus claims that everything is in motion. Plato agrees that all sensible objects are in constant flux, but to him sensible objects are not all things. Plato, Heraclitus, and Protagoras all agree that no contrary can exist if it is separated from its own contrary.

Heraclitean doctrine states that everything is always in the process of becoming, it never “is,” as a result of constant motion. Plato alters this claim that perception is what is in the process of becoming. He interprets the statement, “What I perceive becomes for me,” as meaning that the quantity perceived becomes at the instant it is perceived and only for one percipient. The quality has no independent existence in the physical object at other times.

This is an extension of the Protagorean doctrine. No one else can perceive the object in the same way, and the same person cannot perceive it the same way twice. The change in an object occurs between the object and the sense organ and not within time. The sense organ is no more constant than the object.

Plato now gives what he thinks is a true account of the objects of perception.

Plato intends to refute the claim of perception (in spite of its infallibility) to be knowledge on the ground that its objects have no real being, but are always becoming and changing and therefore cannot be known.2

Plato elaborates on the theory of the nature of sense perception. The objects that are perceived through the senses become perceptible through their motion. Even the perceiving objects produce motion when in the process of perception.

There are two kinds of motion; one is change of place, the motion of the object, and the other is change of quality, the motion of the perceiving subject. These motions come from opposite sources and meet and influence each other. The percipient becomes aware of the effects of friction of these motions, and it experiences a sense quality and the perception of this quality, such as coldness. The constantly changing object meets constant change in the subject and becomes cold or some other quality in the momentary perception of the subject. Two different subjects can never perceive the same qualities in an object, and a subject cannot have two identical perceptions of an object. Theaetetus accepts this account of perception which he has identified with knowledge.

Plato has interpreted Protagoras’ assertion that “what appears to each man is to him” as meaning that what he perceives has meaning for him and that his perception is infallible. Plato’s theory of perception now claims that the object does not have being apart from the percipient and interprets “is to him” as meaning “becomes for him.” Perception is infallible.

Plato then begins to question whether perception as it has been described possesses all the characteristics of knowledge. He also criticizes the aspects of the Heraclitean and Protagorean doctrines which go beyond what he has accepted. Two objections are made against Protagoras’ man-measure theory. One is: Why is man the measure of all things? Why not some other animal, since man has no privileged position with regard to mere sensation? Second, if what everyone believes as the result of perception is true for him, how can any person be wiser than another?

The simple identification of perceiving and knowing is criticized. These criticisms take the meaning of perception in its narrowest form, and although they take Theaetetus’ statement more literally than he meant, they point out the various meanings of the word “know.” Perception must include awareness of memory objects or the identification of perception with knowledge becomes ridiculous. If a man sees and acquires knowledge of what he has seen and then closes his eyes, he remembers the thing but does not see it. But if he does not see it, he does not know it since he does not perceive it. So a man who has come to know something and remembers it does not know it because he does not see it. This is absurd. “I know” must have other meanings than “I am perceiving.”

In order to make the argument more fair, Socrates attempts to defend Protagoras’ theory as he probably would have done himself. Protagoras replies to the objection that a person can know and not know the same thing if he remembers it but does not see it. He claims that the memory image is not the same thing as a present sense-impression. Therefore it is not true that a person can know (remember) and not know (see) the same thing. The objection merely determined that the meaning of perception must be expanded to include awareness of memory images.

Another objection was that if sense-perception and knowledge are identical, a person, who looks at an object with one eye while he has the other shut, knows the object and does not know it. Protagoras can reply to this by saying that the person, who has the conflicting states of consciousness, changes with every sense impression he experiences. Since a person is different with every experience, there is no conflict.

The main objection against his position was that if each man is the measure of his own judgments or beliefs, how can one man be wiser than another? Although every opinion is true, since man is the measure of all things, Protagoras believed that some opinions are better than others, and that his responsibility as an educator was to substitute better opinions for worse ones. He cannot substitute truer beliefs for less true ones, but only sounder ones. Sounder beliefs are those that will produce better results in the future, results which will seem better to the person in the future.

Protagoras’ doctrine, Heraclitean flux, and Theaetetus’ claim that perception is identical to knowledge still have not been seriously criticized. First, Protagoras’ doctrine, every judgment is true for him who makes it, is refuted. Secondly, the Heraclitean theory, all things are in motion, is refuted as fatal to all communication. Finally, the identification of perception with knowledge is denounced.

Plato attacks the doctrine that man is the measure, beyond immediate perception, of all judgments. By Protagoras’ claim that a wiser man has better opinions than another man, the measure of wisdom or ignorance is the result of existing or insufficient knowledge of the useful. There may be momentary difference of opinion whether a thing is useful or harmful. Later the issue is decided unanimously by the consequences which have occurred in the meantime. The wise man can foresee what will happen later with reference to all judgments in the future. The foolish man is one who cannot predict what will happen. When there is a prediction that one plan will produce better effects than another, it is a matter of opinion whether this prediction is true until the results occur.

Protagoras surely thought that his educational methods were truer predictions of the future than those of other teachers. He assumes that his opinion is better than that of others. Most people believe that Protagoras’ theory is false, and, since he concedes that their opinion is true (because all opinions are true), he admits that his theory is false. Although he admits that their opinion is true, if only for them, they do not admit that his theory is true, even for him. Since all opinion is not even true for him, Protagoras is contradicting himself. Plato shows that not all judgments can be true.

[At this point, there is a section of missing material dealing with Plato’s objections to Heraclitus. This lost section contains the references of endnotes 3 and 4. –P. McW.]

Plato does not discuss in the dialogue the doctrine of Parmenides that all motion and change is an illusion, which is at the other extreme from the Heraclitean principle. The criticism of Parmenides is included in the Sophist where the world of unchanging reality appears.

The elements in Protagoras’ doctrine and in Heracliteanism which Plato will not accept have been eliminated. The ones that he accepts and has included in his theory of the nature of perception remain. He now considers Theaetetus’ claim that perception is knowledge, which means that it is the whole of knowledge. He argues that perception cannot be the whole of knowledge because much of what is always called knowledge includes truths involving terms which are not objects of perception. In addition to the separate sense organs there must be a mind which receives their reports and makes judgments on them.

In making these judgments the thinking mind uses terms like “exists,” “is the same as,” “is different from,” which are not objects of perception reaching the mind through the channel of any special sense, but are “common” to all the objects of sense. The mind gains its acquaintance with the meaning of such terms through its own instrumentality, not by the commerce between bodily organs and objects.5

Plato concludes that perception is not the whole of knowledge. His second conclusion is that even within its own sphere, perception is not knowledge at all. The objects of perception do not have the true reality which the objects of knowledge must have. Although perception is infallible, which is the first characteristic of knowledge, it cannot apprehend existence and truth, the second mark of knowledge. The simplest judgment is beyond the scope of perception.

The advance to knowledge is a gradual recovery of clear vision, possible only by a training in dialectic.6

Knowledge, to Plato, has the real for its objects, and the objects of perception do not have true and permanent being. Plato has disproved the claim that perception is knowledge.

Since it has been concluded that knowledge is not perception, Theaetetus suggests that knowledge be defined as true judgment or right opinion. He has realized that knowledge must be above the level of mere sensation; thinking or judgment must be involved. Judgments can be true or false, and he claims that any true judgment is knowledge.

Socrates then asks how false judgment is possible. If we agree that anything is either known or unknown to us, it is difficult to see how we can ever think that one thing, whether we know it or not, can be another thing, whether we know it or not, in other words, mistake one thing for another. As long as the question of false judgment is restricted to these narrow limits, it cannot be explained.

Socrates also objects to identifying false judgment as thinking the thing that is not. When a man thinks, he thinks something, a thing that is. To think what is not is to think nothing, which is the same as not to think at all. Therefore it is impossible to think what is not. Thinking what is not is something different from thinking falsely. False judgment cannot be defined along these lines.

In the dialogue Plato equates the act of mistaking one thing for another with making the silent statement that one thing is the other. If we have both things clearly before our minds, we never judge that one thing is another. The idea of mistaking one thing for another does not explain how we can make a false judgment, if it is assumed that the things must either be known (present to the mind) or unknown (absent from the mind).

To avoid this objection, the contents of the memory are introduced into the argument. The memory is represented as a wax tablet on which impressions of the sense stimuli and the consideration of the stimuli are made. If the impressions are not clear or become distorted when new impressions are made on the tablet, it is possible to mistake them or to read them incorrectly. Error comes, not in the act of perception, but in judgments made about what is perceived. Only a small number of false judgments can be defined as misfitting perception to thought. This is not a definition of false judgment in general; it does not include cases where perception is not involved.

In order to illustrate such cases, the memory is compared to a bird-cage in which all our ideas are confined. The birds flying around in the cage are like pieces of knowledge. A person can possess knowledge without having it around him. It is impossible to possess what one does not possess, so a man cannot know what he does not know. He can get hold of a false judgment about something by catching a different piece of knowledge as he might catch a wrong kind of bird. When he gets hold of the right piece about it, he thinks what is true and has a true judgment about it. False judgment is the interchange of pieces of knowledge.

However it is difficult to explain how a person can fail to recognize a false belief as false and mistake it for a true belief which is stored in his mind. Socrates’ arguments result in the question,

How can I know what I know? How can I recognize knowledge when I have it and be sure that it is knowledge?7

The term “knowledge” is very ambiguous, and, until all its meanings have been revealed, false judgment cannot really be explained. Socrates’ discussion has pointed out some of its meaning. Plato’s analysis of false judgment is included in the Sophist when the Forms have been introduced.

The “pieces of knowledge” stored in the mind are no more than true beliefs. Our attitude toward a false belief is the same as it is toward a true one. Our confidence in belief is not based on reason. Socrates contrasts a jury’s second-hand belief when convinced of the facts to the direct knowledge of the eye-witness who has seen the fact. Even if the jury finds the right verdict, they are still judging without knowledge, only belief. If true belief and knowledge were the same, a juryman could never have a correct belief without knowledge. Therefore, knowledge cannot be defined as true belief.

True belief lacked something which was necessary in order to call it knowledge. So Theaetetus suggests that knowledge is true belief accompanied by an account or explanation. Plato considers the various possible meanings of “account” and finally rejects the suggestion. The account is not enough to raise correct opinion to the level of knowledge.

Socrates considers a theory that he has heard which claims that there are certain elements, the fundamentals of nature, and all other things are complexes of these elements. We have the simple direct perception of the elements, but we do not have knowledge of them. At first we have a true notion without an account of the complex. When we have enumerated the elements of the complex, we have given an account of it and know it. The only things the theory considers as the objects of cognition are concrete individual things. If we enumerate the parts of a complex thing, we reach the elements which cannot be explained and therefore cannot be known. The process of acquiring knowledge is the process of analyzing a complex which is not known into its parts which cannot be known.

However if the letters of a word are considered to be the elements of a syllable, our knowledge of the elements or letters is usually clearer than our knowledge of syllables. The theory, though, regards our perception of elements as inferior to the knowledge we supposedly gain by giving an account of the complex.

Then, if we are to argue from our own experience of elements and complexes to other cases, we shall conclude that elements in general yield knowledge that is much clearer than knowledge of the complex and more effective for a complete grasp of anything we seek to know. If anyone tells us that the complex is by its nature knowable, while the elements unknowable, we shall suppose that, whether he intends it or not, he is playing with us.8

There are three possible meanings of “account,” and none of them succeeds in converting belief into knowledge. It is still assumed that the only things that can be known are concrete individual things and that knowledge is the result of giving an account of such things.

One of the possible meanings of account is the expression of thought in speech. Anyone who is not deaf or mute can speak, and therefore anyone who has a correct notion can express it in speech, in other words, can give an account of it. So there can be no correct notion apart from knowledge.

The second possible meaning is the enumeration of the elementary parts. This meaning is considered without believing that the element must be unknowable. We may have a correct belief about every letter in a name and write it correctly, but not have the assured knowledge that would prevent us from writing it incorrectly another time. If we do not understand it, although we have a correct belief about its parts, we do not know it.

The third possible meaning of account is the statement of a distinguishing mark. When we grasp how an object differs from all other things, we gain knowledge of what we previously had a true notion about. However, Socrates says this means that when we have a right notion of the way things are different from other things, we are to add a correct notion of the way they differ from other things, which is ridiculous. If having an account of something means we must know the differences rather than just having a notion of it, the knowledge is correct belief with a knowledge of differentness, which is absurd. We cannot gain knowledge which is believed to be superior to beliefs, by adding an account in any of the senses that have been considered. All these attempts to define knowledge have failed.

The necessary conclusion from the dialogue’s discussion is that true knowledge has a different kind of things as its objects. It is Plato’s view that

…the objects of which knowledge must give an account are not concrete individuals but objects of thought, and that the simpler terms in which the account must be stated are not material parts but higher concepts.9

The objects of knowledge are not sensible things, but Ideas and truths about them. These objects, which are not discussed in the Theaetetus, do not change in any respect; they had no beginning and will have no end. Therefore it is possible for us to know them. The Theaetetus leads to the conclusion that reality must be incorporeal by illustrating the failure of all attempts to gain knowledge from sensible objects. The dialogue has been an unsuccessful attempt to define knowledge.

1 Cornford, Francis M., Plato’s Theory of Knowledge, 30.

2 Ibid., 49.

3 Ibid., 92.

4 Ibid., 99.

5 Ibid., 105.

6 Ibid., 108.

7 Ibid., 140.

8 Ibid., 154.

9 Ibid.


Clark, Gordon H., Thales to Dewey: A History of Philosophy, Houghton Mifflin Company,
Boston, 1957.

Collier’s Encyclopedia, Vol. 16, P.F. Collier & Son Corporation, New York, 1958.

Cornford, Francis M., Plato’s Theory of Knowledge, Harcourt, Brace & Co., New York, 1935.

Grene, Marjorie, The Knower and the Known, Basic Books, Inc., New York, 1966.

Grube, G.M.A., Plato’s Thought, Beacon Press, Boston, 1966.

Ritter, Constantin, The Essence of Plato’s Philosophy, The Dial Press, New York, 1933.

Taylor, A.E., Plato: The Man and His Work, Methuen & Co., Ltd., London, 1955.

Thilly, Frank, A History of Philosophy, Henry Holt & Company, New York, 1940.