Epictetus, Golden Sayings 99

If a man has frequent intercourse with others, either in the way of conversation, entertainment, or simple familiarity, he must either become like them, or change them to his own fashion. A live coal placed next a dead one will either kindle that or be quenched by it. 

Such being the risk, it is well to be cautious in admitting intimacies of this sort, remembering that one cannot rub shoulders with a soot-stained man without sharing the soot oneself. What will you do, supposing the talk turns on gladiators, or horses, or prize-fighters, or (what is worse) on persons, condemning this and that, approving the other? Or suppose a man sneers and jeers or shows a malignant temper? 

Has any among us the skill of the lute-player, who knows at the first touch which strings are out of tune and sets the instrument right? Has any of you such power as Socrates had, in all his intercourse with men, of winning them over to his own convictions? 

No, but you must be swayed here and there by the uninstructed. How comes it then that they prove so much stronger than you? Because they speak from the fullness of the heart—their low, corrupt views are their real convictions, whereas your fine sentiments are but from the lips, outwards. 

That is why they are so nerveless and dead. It turns one's stomach to listen to your exhortations, and hear of your miserable Virtue, that you prate of up and down. Thus it is that the Vulgar prove too strong for you. Everywhere strength, everywhere victory holds back your conviction!