SOC. And, indeed, if I may venture to say so, it is not a bad description of knowledge

 
that you have given, but one which Protagoras also used to give. Only, he has said the same thing in a different way. For he says somewhere that man is the measure of all things, of the existence of the things that are and the non-existence of the things that are not. You have read that, I suppose? THEAET. Yes, I have read it often. SOC. Well, is not this about what he means, that individual things are for me such as they appear to me, and for you in turn such as they appear to you —you and I being man ? THEAET. Yes, that is what he says. SOC. It is likely that a wise man is not talking nonsense; so let us follow after him. Is it not true that sometimes, when the same wind blows, one of us feels cold, and the other does not? or one feels slightly and the other exceedingly cold? THEAET. Certainly. SOC. Then in that case, shall we say that the wind is in itself cold or not cold or shall we accept Protagoras’s saying that it is cold for him who feels cold and not for him who does not? THEAET. Apparently we shall accept that. SOC. Then it also seems cold, or not, to each of the two? THEAET. Yes. SOC. But seems denotes perceiving? THEAET. It does. SOC. Then seeming and perception are the same thing in matters of warmth and everything of that sort. For as each person perceives things, such they are to each person. THEAET. Apparently. SOC. Perception, then, is always of that which exists and, since it is knowledge, cannot be false. THEAET. So it seems. SOC. By the Graces! I wonder if Protagoras, who was a very wise man, did not utter this dark saying to the common herd like ourselves, and tell the truth An allusion to the title of Protagoras’s book, Truth . in secret to his pupils. THEAET. Why, Socrates, what do you mean by that? SOC. I will tell you and it is not a bad description, either, that nothing is one and invariable, and you could not rightly ascribe any quality whatsoever to anything, but if you call it large it will also appear to be small, and light if you call it heavy, and everything else in the same way, since nothing whatever is one, either a particular thing or of a particular quality; but it is out of movement and motion and mixture with one another that all those things become which we wrongly say are —wrongly, because nothing ever is, but is always becoming. 
 
And on this subject all the philosophers, except Parmenides, may be marshalled in one line—Protagoras and Heracleitus and Empedocles—and the chief poets in the two kinds of poetry, Epicharmus, in comedy, and in tragedy, Homer, who, in the line 0ceanus the origin of the gods, and Tethys their mother, Hom. Il. 14.201, 302 has said that all things are the offspring of flow and motion; or don’t you think he means that? THEAET. I think he does.