"Man Is The Measure Of All Things"

Context: In the dialogue known as Theaetetus, Socrates and Theaetetus discuss the nature of knowledge, seeking to find the true definition of it. After some false starts at answering by his friend, Socrates once again puts his question, "What is knowledge?" Theaetetus begins again by stating that knowledge is perception. Socrates notes that this doctrine is an old and honorable one that was expounded by the famous Protagoras, a Greek philosopher of an earlier generation. Socrates notes, too, that the obvious meaning for Protagoras' statement is that things are actually what they appear to be at any given moment in time. Such a statement, suggests Socrates, is true only in some cases; that Protagoras must have had a deeper meaning. He suggests that Protagoras meant that all things are relative and in motion. Socrates adds that many of the ancients, including Heracleitus and Empedocles, were in agreement with such a statement. Both Socrates and Plato, however, do not believe in the relativity of truth and knowledge. As philosophical idealists they believe in ultimate, immutable, and absolute truth. Plato has Socrates go on to question Theaetetus until both are satisfied that even the statement of such a great thinker as Protagoras is wrong when it attempts to exchange transient and mutable perception for knowledge of truth. Following Socrates' question, asking for a statement of the nature of knowledge, Theaetetus begins:


THEAETETUS
. . . 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 nonexistence 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.