EPISTLE VII: On Crowds
The young character… must be rescued from the mob; it is too easy to side with the majority. Even Socrates, Cato, and Laelius might have been shaken in their moral strength by a crowd that was unlike them; so true it is that none of us, no matter how much he cultivates his abilities, can withstand the shock of faults that approach, as it were, with so great a retinue. Much harm is done by a single case of indulgence or greed; the familiar friend, if he be luxurious, weakens and softens us imperceptibly; the neighbour, if he be rich, rouses our covetousness; the companion, if he be slanderous, rubs off some of his rust upon us, even though we be spotless and sincere. What then do you think the effect will be on character, when the world at large assaults it! You must either imitate or loathe the world. But both courses are to be avoided; you should not copy the bad simply because they are many, nor should you hate the many because they are unlike you. Withdraw into yourself, as far as you can. Associate with those who will make a better man of you. Welcome those whom you yourself can improve. The process is mutual; for men learn while they teach.
Seneca Epistles 1-65, Translation by Richard Gummere. Loeb Classical Library.