Soc. 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 non-existence of things that are not : — You have read him ?
Theaet. O yes, again and again.
Soc. 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 ?
Theaet. Yes, he says so.
Soc. A wise man is not likely to talk nonsense. Let us try to understand him : the same wind is blowing, and yet one of us may be cold and the other not, or one may be slightly and the other very cold ?
Theaet. Quite true.
Soc. Now is the wind, regarded not in relation to us but absolutely, cold or not ; or are we to say, with Protagoras, that the wind is cold to him who is cold, and not to him who is not ?
Theaet. I suppose the last.
Soc. Then it must appear so to each of them ?
Theaet. Yes.
Soc. And “appears to him” means the same as “he perceives.”
Theaet. True.
Soc. Then appearing and perceiving coincide in the case of hot and cold, and in similar instances ; for things appear, or may be supposed to be, to each one such as he perceives them ?
Theaet. Yes.
Soc. Then perception is always of existence, and being the same as knowledge is unerring ?
Theaet. Clearly.