Discourses, "Of Progress or Improvement" (§1)

“He who is making progress, having learned from philosophers that desire means the desire of good things, and aversion means aversion from bad things; having learned too that happiness and tranquillity are not attainable by man otherwise than by not failing to obtain what he desires, and not falling into that which he would avoid; such a man takes from himself desire altogether and confers it, but he employs his aversion only on things which are dependent on his will. For if he attempts to avoid anything independent of his will, he knows that sometimes he will fall in with something which he wishes to avoid, and he will be unhappy.”