trans. Jowett (1871)
I replied: I will begin again at the same point, Protagoras, and tell you once more the purport of my visit: this is my friend Hippocrates, who is desirous of making your acquaintance; he would like to know what will happen to him if he associates with you. I have no more to say.
Protagoras answered: Young man, if you associate with me, on the very first day you will return home a better man than you came, and better on the second day than on the first, and better every day than you were on the day before.
When I heard this, I said: Protagoras, I do not at all wonder at hearing you say this; even at your age, and with all your wisdom, if any one were to teach you what you did not know before, you would become better no doubt: but please to answer in a different way-I will explain how by an example. Let me suppose that Hippocrates, instead of desiring your acquaintance, wished to become acquainted with the young man Zeuxippus of Heraclea, who has lately been in Athens, and he had come to him as he has come to you, and had heard him say, as he has heard you say, that every day he would grow and become better if he associated with him: and then suppose that he were to ask him, “In what shall I become better, and in what shall I grow?”-Zeuxippus would answer, “In painting.” And suppose that he went to Orthagoras the Theban, and heard him say the same thing, and asked him, “In what shall I become better day by day?” he would reply, “In flute-playing.” Now I want you to make the same sort of answer to this young man and to me, who am asking questions on his account. When you say that on the first day on which he associates with you he will return home a better man, and on every day will grow in like manner,-In what, Protagoras, will he be better? and about what?
When Protagoras heard me say this, he replied: You ask questions fairly, and I like to answer a question which is fairly put. If Hippocrates comes to me he will not experience the sort of drudgery with which other Sophists are in the habit of insulting their pupils; who, when they have just escaped from the arts, are taken and driven back into them by these teachers, and made to learn calculation, and astronomy, and geometry, and music (he gave a look at Hippias as he said this); but if he comes to me, he will learn that which he comes to learn.
Compare W.R. Lamb’s 1924 translation at the Perseus Project (Greek text and commentary also available).