Selections from Plato's Theaetetus Concerning Knowledge as Perception

UC Davis Philosophy 102

Theory of Knowledge

Fall, 2004

Instructor: G. J. Mattey, Senior Lecturer


Selections are taken from the public-domain translation of Plato's Theaeteus by Benjamin Jowett.


What is Knowledge? (146a-147d)

SOCRATES: Herein lies the difficulty which I can never solve to my satisfaction--What is knowledge? Can we answer that question? What say you? which of us will speak first? whoever misses shall sit down, as at a game of ball, and shall be donkey, as the boys say; he who lasts out his competitors in the game without missing, shall be our king, and shall have the right of putting to us any questions which he pleases...Why is there no reply? I hope, Theodorus, that I am not betrayed into rudeness by my love of conversation? I only want to make us talk and be friendly and sociable.
THEODORUS: The reverse of rudeness, Socrates: but I would rather that you would ask one of the young fellows; for the truth is, that I am unused to your game of question and answer, and I am too old to learn; the young will be more suitable, and they will improve more than I shall, for youth is always able to improve. And so having made a beginning with Theaetetus, I would advise you to go on with him and not let him off.
SOCRATES: Do you hear, Theaetetus, what Theodorus says? The philosopher, whom you would not like to disobey, and whose word ought to be a command to a young man, bids me interrogate you. Take courage, then, and nobly say what you think that knowledge is.
THEAETETUS: Well, Socrates, I will answer as you and he bid me; and if I make a mistake, you will doubtless correct me.
SOCRATES: We will, if we can.
THEAETETUS: Then, I think that the sciences which I learn from Theodorus-- geometry, and those which you just now mentioned--are knowledge; and I would include the art of the cobbler and other craftsmen; these, each and all of, them, are knowledge.
SOCRATES: Too much, Theaetetus, too much; the nobility and liberality of your nature make you give many and diverse things, when I am asking for one simple thing.
THEAETETUS: What do you mean, Socrates?
SOCRATES: Perhaps nothing. I will endeavour, however, to explain what I believe to be my meaning: When you speak of cobbling, you mean the art or science of making shoes?
THEAETETUS: Just so.
SOCRATES: And when you speak of carpentering, you mean the art of making wooden implements?
THEAETETUS: I do.
SOCRATES: In both cases you define the subject matter of each of the two arts?
THEAETETUS: True.
SOCRATES: But that, Theaetetus, was not the point of my question: we wanted to know not the subjects, nor yet the number of the arts or sciences, for we were not going to count them, but we wanted to know the nature of knowledge in the abstract. Am I not right?
THEAETETUS: Perfectly right.
SOCRATES: Let me offer an illustration: Suppose that a person were to ask about some very trivial and obvious thing--for example, What is clay? and we were to reply, that there is a clay of potters, there is a clay of oven- makers, there is a clay of brick-makers; would not the answer be ridiculous?
THEAETETUS: Truly.
SOCRATES: In the first place, there would be an absurdity in assuming that he who asked the question would understand from our answer the nature of 'clay,' merely because we added 'of the image-makers,' or of any other workers. How can a man understand the name of anything, when he does not know the nature of it?
THEAETETUS: He cannot.
SOCRATES: Then he who does not know what science or knowledge is, has no knowledge of the art or science of making shoes?
THEAETETUS: None.
SOCRATES: Nor of any other science?
THEAETETUS: No.
SOCRATES: And when a man is asked what science or knowledge is, to give in answer the name of some art or science is ridiculous; for the question is, 'What is knowledge?' and he replies, 'A knowledge of this or that.'
THEAETETUS: True.
SOCRATES: Moreover, he might answer shortly and simply, but he makes an enormous circuit. For example, when asked about the clay, he might have said simply, that clay is moistened earth--what sort of clay is not to the point.
THEAETETUS: Yes, Socrates, there is no difficulty as you put the question.

Knowledge is Perception (151d-152c)

THEAETETUS: At any rate, Socrates, after such an exhortation I should be ashamed of not trying to do my best. Now he who knows perceives what he knows, and, as far as I can see at present, knowledge is perception.
SOCRATES: Bravely said, boy; that is the way in which you should express your opinion. And now, let us examine together this conception of yours, and see whether it is a true birth or a mere wind-egg:--You say that knowledge is perception?
THEAETETUS: Yes.
SOCRATES: Well, you have delivered yourself of a very important doctrine about knowledge; it is indeed the opinion of Protagoras, who has another way of expressing it. Man, he says, is the measure of all things, of the existence of things that are, and of the non-existence of things that are not:--You have read him?
THEAETETUS: O yes, again and again.
SOCRATES: Does he not say that things are to you such as they appear to you, and to me such as they appear to me, and that you and I are men?
THEAETETUS: Yes, he says so.
SOCRATES: A wise man is not likely to talk nonsense. Let us try to understand him: the same wind is blowing, and yet one of us may be cold and the other not, or one may be slightly and the other very cold?
THEAETETUS: Quite true.
SOCRATES: Now is the wind, regarded not in relation to us but absolutely, cold or not; or are we to say, with Protagoras, that the wind is cold to him who is cold, and not to him who is not?
THEAETETUS: I suppose the last.
SOCRATES: Then it must appear so to each of them?
THEAETETUS: Yes.
SOCRATES: And 'appears to him' means the same as 'he perceives.'
THEAETETUS: True.
SOCRATES: Then appearing and perceiving coincide in the case of hot and cold, and in similar instances; for things appear, or may be supposed to be, to each one such as he perceives them?
THEAETETUS: Yes.
SOCRATES: Then perception is always of existence, and being the same as knowledge is unerring?
THEAETETUS: Clearly.

Dreams and Disorders (157e-158d)

SOCRATES: Let us not leave the argument unfinished, then; for there still remains to be considered an objection which may be raised about dreams and diseases, in particular about madness, and the various illusions of hearing and sight, or of other senses. For you know that in all these cases the esse-percipi theory appears to be unmistakably refuted, since in dreams and illusions we certainly have false perceptions; and far from saying that everything is which appears, we should rather say that nothing is which appears.
THEAETETUS: Very true, Socrates.
SOCRATES: But then, my boy, how can any one contend that knowledge is perception, or that to every man what appears is?
THEAETETUS: I am afraid to say, Socrates, that I have nothing to answer, because you rebuked me just now for making this excuse; but I certainly cannot undertake to argue that madmen or dreamers think truly, when they imagine, some of them that they are gods, and others that they can fly, and are flying in their sleep.
SOCRATES: Do you see another question which can be raised about these phenomena, notably about dreaming and waking?
THEAETETUS: What question?
SOCRATES: A question which I think that you must often have heard persons ask:--How can you determine whether at this moment we are sleeping, and all our thoughts are a dream; or whether we are awake, and talking to one another in the waking state?
THEAETETUS: Indeed, Socrates, I do not know how to prove the one any more than the other, for in both cases the facts precisely correspond;--and there is no difficulty in supposing that during all this discussion we have been talking to one another in a dream; and when in a dream we seem to be narrating dreams, the resemblance of the two states is quite astonishing.
SOCRATES: You see, then, that a doubt about the reality of sense is easily raised, since there may even be a doubt whether we are awake or in a dream. And as our time is equally divided between sleeping and waking, in either sphere of existence the soul contends that the thoughts which are present to our minds at the time are true; and during one half of our lives we affirm the truth of the one, and, during the other half, of the other; and are equally confident of both.
THEAETETUS: Most true.
SOCRATES: And may not the same be said of madness and other disorders? the difference is only that the times are not equal.
THEAETETUS: Certainly.

Knowledge by Memory (163a-164c)

SOCRATES: Then you and Theodorus mean to say that we must look at the matter in some other way?
THEAETETUS: Yes, in quite another way.
SOCRATES: And the way will be to ask whether perception is or is not the same as knowledge; for this was the real point of our argument, and with a view to this we raised (did we not?) those many strange questions.
THEAETETUS: Certainly.
SOCRATES: Shall we say that we know every thing which we see and hear? for example, shall we say that not having learned, we do not hear the language of foreigners when they speak to us? or shall we say that we not only hear, but know what they are saying? Or again, if we see letters which we do not understand, shall we say that we do not see them? or shall we aver that, seeing them, we must know them?
THEAETETUS: We shall say, Socrates, that we know what we actually see and hear of them--that is to say, we see and know the figure and colour of the letters, and we hear and know the elevation or depression of the sound of them; but we do not perceive by sight and hearing, or know, that which grammarians and interpreters teach about them.
SOCRATES: Capital, Theaetetus; and about this there shall be no dispute, because I want you to grow; but there is another difficulty coming, which you will also have to repulse.
THEAETETUS: What is it?
SOCRATES: Some one will say, Can a man who has ever known anything, and still has and preserves a memory of that which he knows, not know that which he remembers at the time when he remembers? I have, I fear, a tedious way of putting a simple question, which is only, whether a man who has learned, and remembers, can fail to know?
THEAETETUS: Impossible, Socrates; the supposition is monstrous.
SOCRATES: Am I talking nonsense, then? Think: is not seeing perceiving, and is not sight perception?
THEAETETUS: True.
SOCRATES: And if our recent definition holds, every man knows that which he has seen?
THEAETETUS: Yes.
SOCRATES: And you would admit that there is such a thing as memory?
THEAETETUS: Yes.
SOCRATES: And is memory of something or of nothing?
THEAETETUS: Of something, surely.
SOCRATES: Of things learned and perceived, that is?
THEAETETUS: Certainly.
SOCRATES: Often a man remembers that which he has seen?
THEAETETUS: True.
SOCRATES: And if he closed his eyes, would he forget?
THEAETETUS: Who, Socrates, would dare to say so?
SOCRATES: But we must say so, if the previous argument is to be maintained.
THEAETETUS: What do you mean? I am not quite sure that I understand you, though I have a strong suspicion that you are right.
SOCRATES: As thus: he who sees knows, as we say, that which he sees; for perception and sight and knowledge are admitted to be the same.
THEAETETUS: Certainly.
SOCRATES: But he who saw, and has knowledge of that which he saw, remembers, when he closes his eyes, that which he no longer sees.
THEAETETUS: True.
SOCRATES: And seeing is knowing, and therefore not-seeing is not-knowing?
THEAETETUS: Very true.
SOCRATES: Then the inference is, that a man may have attained the knowledge of something, which he may remember and yet not know, because he does not see; and this has been affirmed by us to be a monstrous supposition.
THEAETETUS: Most true.
SOCRATES: Thus, then, the assertion that knowledge and perception are one, involves a manifest impossibility?
THEAETETUS: Yes.
SOCRATES: Then they must be distinguished?
THEAETETUS: I suppose that they must.

Knowledge and Reasoning (184b-186e)

SOCRATES: Then now, Theaetetus, take another view of the subject: you answered that knowledge is perception?
THEAETETUS: I did.
SOCRATES: And if any one were to ask you: With what does a man see black and white colours? and with what does he hear high and low sounds?--you would say, if I am not mistaken, 'With the eyes and with the ears.'
THEAETETUS: I should.
SOCRATES: The free use of words and phrases, rather than minute precision, is generally characteristic of a liberal education, and the opposite is pedantic; but sometimes precision is necessary, and I believe that the answer which you have just given is open to the charge of incorrectness; for which is more correct, to say that we see or hear with the eyes and with the ears, or through the eyes and through the ears.
THEAETETUS: I should say 'through,' Socrates, rather than 'with.'
SOCRATES: Yes, my boy, for no one can suppose that in each of us, as in a sort of Trojan horse, there are perched a number of unconnected senses, which do not all meet in some one nature, the mind, or whatever we please to call it, of which they are the instruments, and with which through them we perceive objects of sense.
THEAETETUS: I agree with you in that opinion.
SOCRATES: The reason why I am thus precise is, because I want to know whether, when we perceive black and white through the eyes, and again, other qualities through other organs, we do not perceive them with one and the same part of ourselves, and, if you were asked, you might refer all such perceptions to the body. Perhaps, however, I had better allow you to answer for yourself and not interfere. Tell me, then, are not the organs through which you perceive warm and hard and light and sweet, organs of the body?
THEAETETUS: Of the body, certainly.
SOCRATES: And you would admit that what you perceive through one faculty you cannot perceive through another; the objects of hearing, for example, cannot be perceived through sight, or the objects of sight through hearing?
THEAETETUS: Of course not.
SOCRATES: If you have any thought about both of them, this common perception cannot come to you, either through the one or the other organ?
THEAETETUS: It cannot.
SOCRATES: How about sounds and colours: in the first place you would admit that they both exist?
THEAETETUS: Yes.
SOCRATES: And that either of them is different from the other, and the same with itself?
THEAETETUS: Certainly.
SOCRATES: And that both are two and each of them one?
THEAETETUS: Yes.
SOCRATES: You can further observe whether they are like or unlike one another?
THEAETETUS: I dare say.
SOCRATES: But through what do you perceive all this about them? for neither through hearing nor yet through seeing can you apprehend that which they have in common. Let me give you an illustration of the point at issue:--If there were any meaning in asking whether sounds and colours are saline or not, you would be able to tell me what faculty would consider the question. It would not be sight or hearing, but some other.
THEAETETUS: Certainly; the faculty of taste.
SOCRATES: Very good; and now tell me what is the power which discerns, not only in sensible objects, but in all things, universal notions, such as those which are called being and not-being, and those others about which we were just asking--what organs will you assign for the perception of these notions?
THEAETETUS: You are thinking of being and not being, likeness and unlikeness, sameness and difference, and also of unity and other numbers which are applied to objects of sense; and you mean to ask, through what bodily organ the soul perceives odd and even numbers and other arithmetical conceptions.
SOCRATES: You follow me excellently, Theaetetus; that is precisely what I am asking.
THEAETETUS: Indeed, Socrates, I cannot answer; my only notion is, that these, unlike objects of sense, have no separate organ, but that the mind, by a power of her own, contemplates the universals in all things.
SOCRATES: You are a beauty, Theaetetus, and not ugly, as Theodorus was saying; for he who utters the beautiful is himself beautiful and good. And besides being beautiful, you have done me a kindness in releasing me from a very long discussion, if you are clear that the soul views some things by herself and others through the bodily organs. For that was my own opinion, and I wanted you to agree with me.
THEAETETUS: I am quite clear.
SOCRATES: And to which class would you refer being or essence; for this, of all our notions, is the most universal?
THEAETETUS: I should say, to that class which the soul aspires to know of herself.
SOCRATES: And would you say this also of like and unlike, same and other?
THEAETETUS: Yes.
SOCRATES: And would you say the same of the noble and base, and of good and evil?
THEAETETUS: These I conceive to be notions which are essentially relative, and which the soul also perceives by comparing in herself things past and present with the future.
SOCRATES: And does she not perceive the hardness of that which is hard by the touch, and the softness of that which is soft equally by the touch?
THEAETETUS: Yes.
SOCRATES: But their essence and what they are, and their opposition to one another, and the essential nature of this opposition, the soul herself endeavours to decide for us by the review and comparison of them?
THEAETETUS: Certainly.
SOCRATES: The simple sensations which reach the soul through the body are given at birth to men and animals by nature, but their reflections on the being and use of them are slowly and hardly gained, if they are ever gained, by education and long experience.
THEAETETUS: Assuredly.
SOCRATES: And can a man attain truth who fails of attaining being?
THEAETETUS: Impossible.
SOCRATES: And can he who misses the truth of anything, have a knowledge of that thing?
THEAETETUS: He cannot.
SOCRATES: Then knowledge does not consist in impressions of sense, but in reasoning about them; in that only, and not in the mere impression, truth and being can be attained?
THEAETETUS: Clearly.
SOCRATES: And would you call the two processes by the same name, when there is so great a difference between them?
THEAETETUS: That would certainly not be right.
SOCRATES: And what name would you give to seeing, hearing, smelling, being cold and being hot?
THEAETETUS: I should call all of them perceiving--what other name could be given to them?
SOCRATES: Perception would be the collective name of them?
THEAETETUS: Certainly.
SOCRATES: Which, as we say, has no part in the attainment of truth any more than of being?
THEAETETUS: Certainly not.
SOCRATES: And therefore not in science or knowledge?
THEAETETUS: No.
SOCRATES: Then perception, Theaetetus, can never be the same as knowledge or science?
THEAETETUS: Clearly not, Socrates; and knowledge has now been most distinctly proved to be different from perception.


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