Theaetetus (160)

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.