Men are disturbed, not by the things which happen, but by the opinions about the things.

Epictetus taught by speaking, and this line comes from the Enchiridion, the short handbook his pupil Arrian compiled from those lectures. It is the idea the later cognitive therapies took from Stoicism: how we feel depends on how we interpret what happens. The wording is George Long’s 1877 translation, which is in the public domain.