SOC. And all the rest—hard and hot and so forth—must be regarded in the same way: we must assume,

 
we said before, that nothing exists in itself, but all things of all sorts arise out of motion by intercourse with each other; for it is, as they say, impossible to form a firm conception of the active or the passive element as being anything separately; for there is no active element until there is a union with the passive element, nor is there a passive element until there is a union with the active; and that which unites with one thing is active and appears again as passive when it comes in contact with something else. And so it results from all this, as we said in the beginning, that nothing exists as invariably one, itself by itself, but everything is always becoming in relation to something, and being should be altogether abolished,
 
though we have often—and even just now—been compelled by custom and ignorance to use the word. But we ought not, the wise men say, to permit the use of something or somebody’s or mine or this or that or any other word that implies making things stand still, but in accordance with nature we should speak of things as becoming and being made and being destroyed and changing ; for anyone who by his mode of speech makes things stand still is easily refuted. And we must use such expressions in relation both to particular objects and collective designations, among which are
 
 mankind and stone and the names of every animal and class. Do these doctrines seem pleasant to you, Theaetetus, and do you find their taste agreeable? THEAET. I don’t know, Socrates; besides, I can’t tell about you, either, whether you are preaching them because you believe them or to test me. SOC. You forget, my friend, that I myself know nothing about such things, and claim none of them as mine, but am incapable of bearing them and am merely acting as a midwife to you, and for that reason am uttering incantations and giving you a taste of each of the philosophical theories,
 
until I may help to bring your own opinion to light. And when it is brought to light, I will examine it and see whether it is a mere wind-egg or a real offspring. So be brave and patient, and in good and manly fashion tell what you think in reply to my questions. THEAET. Very well; ask them. SOC. Then say once more whether the doctrine pleases you that nothing is, but is always becoming—good or beautiful or any of the other qualities we were just enumerating. THEAET. Why, when I hear you telling about it as you did, it seems to me that it is wonderfully reasonable and ought to be accepted as you have presented it. SOC. Let us, then, not neglect a point in which it is defective. The defect is found in connection with dreams and diseases, including insanity, and everything else that is said to cause illusions of sight and hearing and the other senses. For of course you know that in all these the doctrine we were just presenting seems admittedly to be refuted, because

 
in them we certainly have false perceptions, and it is by no means true that everything is to each man which appears to him; on the contrary, nothing is which appears. THEAET. What you say is very true, Socrates. SOC. What argument is left, then, my boy, for the man who says that perception is knowledge and that in each case the things which appear are to the one to whom they appear? THEAET. I hesitate to say, Socrates, that I have no reply to make, because you scolded me just now when I said that. 
 
But really I cannot dispute that those who are insane or dreaming have false opinions, when some of them think they are gods and others fancy in their sleep that they have wings and are flying. SOC. Don’t you remember, either, the similar dispute about these errors, especially about sleeping and waking? THEAET. What dispute? SOC. One which I fancy you have often heard. The question is asked, what proof you could give if anyone should ask us now, at the present moment, whether we are asleep and our thoughts are a dream, or whether we are awake
 
and talking with each other in a waking condition. THEAET. Really, Socrates, I don’t see what proof can be given; for there is an exact correspondence in all particulars, as between the strophe and antistrophe of a choral song. Take, for instance, the conversation we have just had: there is nothing to prevent us from imagining in our sleep also that we are carrying on this conversation with each other, and when in a dream we imagine that we are relating dreams, the likeness between the one talk and the other is remarkable. SOC. So you see it is not hard to dispute the point, since it is even open to dispute whether we are awake or in a dream. 
 
Now since the time during which we are asleep is equal to that during which we are awake, in each state our spirit contends that the semblances that appear to it at any time are certainly true, so that for half the time we say that this is true, and for half the time the other, and we maintain each with equal confidence. THEAET. Certainly. SOC. And may not, then, the same be said about insanity and the other diseases, except that the time is not equal? THEAET. Yes. SOC. Well, then, shall truth be determined by the length or shortness of time? THEAET. That would be absurd in many ways. SOC. But can you show clearly in any other way which of the two sets of opinions is true? THEAET. I do not think I can. SOC. Listen, then, while I tell you what would be said about them by those who maintain that what appears at any time is true for him to whom it appears. They begin, I imagine, by asking this question: Theaetetus, can that which is wholly other have in any way the same quality as its alternative? And we must not assume that the thing in question is partially the same and partially other, but wholly other. THEAET. It is impossible for it to be the same in anything, either in quality

 
or in any other respect whatsoever, when it is wholly other. SOC. Must we not, then, necessarily agree that such a thing is also unlike? THEAET. It seems so to me. SOC. Then if anything happens to become like or unlike anything—either itself or anything else—we shall say that when it becomes like it becomes the same, and when it becomes unlike it becomes other? THEAET. We must. SOC. Well, we said before, did we not, that the active elements were many—infinite in fact—and likewise the passive elements? THEAET. Yes. SOC. And furthermore, that any given element, by uniting at different times with different partners, will beget, not the same, but other results? THEAET. Certainly. SOC. Well, then, let us take me, or you, or anything else at hand, and apply the same principle—say Socrates in health and Socrates in illness. Shall we say the one is like the other, or unlike? THEAET. When you say Socrates in illness do you mean to compare that Socrates as a whole with Socrates in health as a whole? SOC. You understand perfectly; that is just what I mean. THEAET. Unlike, I imagine. SOC. And therefore other, inasmuch as unlike? THEAET. Necessarily. SOC. And you would say the same of Socrates asleep or in any of the other states
 
we enumerated just now? THEAET. Yes. SOC. Then each of those elements which by the law of their nature act upon something else, will, when it gets hold of Socrates in health, find me one object to act upon, and when it gets hold of me in illness, another? THEAET. How can it help it? SOC. And so, in the two cases, that active element and I, who am the passive element, shall each produce a different object? THEAET. Of course. SOC. So, then, when I am in health and drink wine, it seems pleasant and sweet to me? THEAET. Yes. SOC. The reason is, in fact, that according to the principles we accepted a while ago,
 
the active and passive elements produce sweetness and perception, both of which are simultaneously moving from one place to another, and the perception, which comes from the passive element, makes the tongue perceptive, and the sweetness, which comes from the wine and pervades it, passes over and makes the wine both to be and to seem sweet to the tongue that is in health. THEAET. Certainly, such are the principles we accepted a while ago. SOC. But when it gets hold of me in illness, in the first place, it really doesn’t get hold of the same man, does it? For he to whom it comes is certainly unlike. THEAET. True. SOC. Therefore the union of the Socrates who is ill and the draught of wine produces other results: in the tongue the sensation or perception of bitterness, and in the wine—a bitterness which is engendered there and passes over into the other; the wine is made, not bitterness, but bitter, and I am made, not perception, but perceptive. THEAET. Certainly. SOC. Then I shall never have this perception of any other thing; for a perception of another thing is another perception,

 
and makes the percipient different and other: nor can that which acts on me ever by union with another produce the same result or become the same in kind; for by producing another result from another passive element it will become different in kind. THEAET. That is true. SOC. And neither shall I, furthermore, ever again become the same as I am, nor will that ever become the same as it is. THEAET. No. SOC. And yet, when I become percipient, I must necessarily become percipient of something, for it is impossible to become percipient and perceive nothing; and that which is perceived must become so to someone,
 
when it becomes sweet or bitter or the like; for to become sweet, but sweet to no one, is impossible. THEAET. Perfectly true. SOC. The result, then, I think, is that we (the active and the passive elements) are or become, whichever is the case, in relation to one another, since we are bound to one another by the inevitable law of our being, but to nothing else, not even to ourselves. The result, then, is that we are bound to one another; and so if a man says anything is, he must say it is to or of or in relation to something, and similarly if he says it becomes ; but he must not say
 
it is or becomes absolutely, nor can he accept such a statement from anyone else. That is the meaning of the doctrine we have been describing. THEAET. Yes, quite so, Socrates. SOC. Then, since that which acts on me is to me and to me only, it is also the case that I perceive it, and I only? THEAET. Of course. SOC. Then to me my perception is true; for in each case it is always part of my being; and I am, as Protagoras says, the judge of the existence of the things that are to me and of the non-existence of those that are not to me. THEAET. So it seems. SOC. How, then, if I am an infallible judge and my mind never stumbles in regard to the things that are or that become, can I fail to know that which I perceive? THEAET. You cannot possibly fail. SOC. Therefore you were quite right in saying that knowledge is nothing else than perception, and there is complete identity between the doctrine of Homer and Heracleitus and all their followers—that all things are in motion, like streams—the doctrine of the great philosopher Protagoras that man is the measure of all things—and the doctrine of Theaetetus that,
 
since these things are true, perception is knowledge. Eh, Theaetetus? Shall we say that this is, so to speak, your new-born child and the result of my midwifery? Or what shall we say? THEAET. We must say that, Socrates. SOC. Well, we have at last managed to bring this forth, whatever it turns out to be; and now that it is born, we must in very truth perform the rite of running round with it in a circle— The rite called amphidromia took place a few days after the birth of a child. After some ceremonies of purification the nurse, in the presence of the family, carried the infant rapidly about the family hearth, thereby introducing him, as it were, to the family and the family deities. At this time the father decided whether to bring up the child or to expose it. Sometimes, perhaps, the child was named on this occasion. In the evening relatives assembled for a feast at which shell-fish were eaten. the circle of our argument—and see whether it may not turn out to be after all not worth rearing, but only a wind-egg,

 
an imposture. But, perhaps, you think that any offspring of yours ought to be cared for and not put away; or will you bear to see it examined and not get angry if it is taken away from you, though it is your first-born? THEO. Theaetetus will bear it, Socrates, for he is not at all ill-tempered. But for heaven’s sake, Socrates, tell me, is all this wrong after all? SOC. You are truly fond of argument, Theodorus, and a very good fellow to think that I am a sort of bag full of arguments and can easily pull one out and say that after all the other one was wrong; 
 
but you do not understand what is going on: none of the arguments comes from me, but always from him who is talking with me. I myself know nothing, except just a little, enough to extract an argument from another man who is wise and to receive it fairly. And now I will try to extract this thought from Theaetetus, but not to say anything myself. THEO. That is the better way, Socrates; do as you say. SOC. Do you know, then, Theodorus, what amazes me in your friend Protagoras? THEO. What is it? SOC. In general I like his doctrine that what appears to each one is to him, but I am amazed by the beginning of his book. I don’t see why he does not say in the beginning of his Truth Truth was apparently the title, or part of the title, of Protagoras’s book. that a pig or a dog-faced baboon or some still stranger creature of those that have sensations is the measure of all things. Then he might have begun to speak to us very imposingly and condescendingly, showing that while we were honoring him like a god for his wisdom, he was after all no better in intellect than any other man,
 
or, for that matter, than a tadpole. What alternative is there, Theodorus? For if that opinion is true to each person which he acquires through sensation, and no one man can discern another’s condition better than he himself, and one man has no better right to investigate whether another’s opinion is true or false than he himself, but, as we have said several times, each man is to form his own opinions by himself, and these opinions are always right and true, why in the world, my friend, was Protagoras wise, so that he could rightly be thought worthy
 
to be the teacher of other men and to be well paid, and why were we ignorant creatures and obliged to go to school to him, if each person is the measure of his own wisdom? Must we not believe that Protagoras was playing to the gallery in saying this? I say nothing of the ridicule that I and my science of midwifery deserve in that case,—and, I should say, the whole practice of dialectics, too. For would not the investigation of one another’s fancies and opinions, and the attempt to refute them, when each man’s must be right, be tedious

 
and blatant folly, if the Truth of Protagoras is true and he was not jesting when he uttered his oracles from the shrine of his book?