Socrates SOC. Then I shall never have this perception of any other thing; for a perception of another thing is another perception, 160a 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.

Theaetetus THEAET. That is true.

Socrates SOC. And neither shall I, furthermore, ever again become the same as I am, nor will that ever become the same as it is.

Theaetetus THEAET. No.

Socrates 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, 160b when it becomes sweet or bitter or the like; for to become sweet, but sweet to no one, is impossible.

Theaetetus THEAET. Perfectly true.

Socrates 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 160c 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.

Theaetetus THEAET. Yes, quite so, Socrates.

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?

Theaetetus THEAET. Of course.

Socrates 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.

Theaetetus THEAET. So it seems.

Socrates 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?

Theaetetus THEAET. You cannot possibly fail.

Socrates 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, 160e 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?

Theaetetus THEAET. We must say that, Socrates.

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— ¹ 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, 161a 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?

Theodorus 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?

Socrates 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; 161b 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.

Theodorus THEO. That is the better way, Socrates; do as you say.

Socrates SOC. Do you know, then, Theodorus, what amazes me in your friend Protagoras?

Theodorus THEO. What is it?

Socrates 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 ² 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, 161d 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 161e 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 162a 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?

Theodorus THEO. Socrates, the man was my friend, as you just remarked. So I should hate to bring about the refutation of Protagoras by agreeing with you, and I should hate also to oppose you contrary to my real convictions. So take Theaetetus again; especially as he seemed just now to follow your suggestions very carefully.

Socrates SOC. If you went to Sparta, Theodorus, 162b and visited the wrestling-schools, would you think it fair to look on at other people naked, some of whom were of poor physique, without stripping and showing your own form, too?

Theodorus THEO. Why not, if I could persuade them to allow me to do so? So now I think I shall persuade you to let me be a spectator, and not to drag me into the ring, since I am old and stiff, but to take the younger and nimbler man as your antagonist.

Socrates SOC. Well, Theodorus, if that pleases you, 162c it does not displease me, as the saying is. So I must attack the wise Theaetetus again. Tell me, Theaetetus, referring to the doctrine we have just expounded, do you not share my amazement at being suddenly exalted to an equality with the wisest man, or even god? Or do you think Protagoras’s “measure” applies any less to gods than to men?

Theaetetus THEAET. By no means; and I am amazed that you ask such a question at all; for when we were discussing the meaning of the doctrine 162d that whatever appears to each one really is to him, I thought it was good; but now it has suddenly changed to the opposite.

Socrates SOC. You are young, my dear boy; so you are quickly moved and swayed by popular oratory. For in reply to what I have said, Protagoras, or someone speaking for him, will say, “Excellent boys and old men, there you sit together declaiming to the people, and you bring in the gods, the question of whose 162e existence or non-existence I exclude from oral and written discussion, and you say the sort of thing that the crowd would readily accept—that it is a terrible thing if every man is to be no better than any beast in point of wisdom; but you do not advance any cogent proof whatsoever; you base your statements on probability. If Theodorus, or any other geometrician, should base his geometry on probability, he would be of no account at all. So you and Theodorus had better consider whether you will accept arguments founded on plausibility and probabilities in 163a such important matters.”