It is common opinion among us in regard to beauty and wisdom that there is an honourable and a shameful way of bestowing them. For to offer one's beauty for money to all comers is called prostitution; but we think it virtuous to become friendly with a lover who is known to be a man of honour. So is it with wisdom. Those who offer it to all comers for money are known as sophists, prostitutors of wisdom.

Words spoken by Socrates to Antiphon in Memorabilia, 1.6.11.