Plato's Apology 19 D – 20 C

But in fact none of these things are true, and if you have heard from anyone that I undertake to teach people and that I make money by it, that is not true either. Although this also seems to me to be a fine thing, if one might be able to teach people, as Gorgias of Leontini and Prodicus of Ceos and Hippias of Elis are. For each of these men, gentlemen, is able to go into any one of the cities and persuade the young men, who can associate for nothing with whomsoever they wish among their own fellow citizens, to give up the association with those men and to associate with them and pay them money and be grateful besides.

And there is also another wise man here, a Parian, who I learned was in town; for I happened to meet a man who has spent more on sophists than all the rest, Callias, the son of Hipponicus; so I asked him -- for he has two sons -- "Callias," said I, "if your two sons had happened to be two colts or two calves, we should be able to get and hire for them an overseer who would make them virtuous in the kind of virtue proper to them; and he would be a horse-trainer or a husbandman; but now, since they are two human beings, whom have you in mind to get as overseer? Who has knowledge of that kind of virtue, that of a man and a citizen? For I think you have looked into the matter, because you have the sons. Is there anyone," said I, "or not? " "Certainly," said he. "Who," said I, "and where from, and what is his price for his teaching? " "Evenus," he said, "Socrates, from Paros, five minae. " And I called Evenus blessed, if he really had this art and taught so reasonably. I myself should be vain and put on airs, if I understood these things; but I do not understand them, men of Athens.