If you have given way to anger, be sure that over and above the evil involved therein, you have strengthened the habit, and added fuel to the fire. If overcome by a temptation of the flesh, do not reckon it a single defeat, but that you have also strengthened your dissolute habits. Habits and faculties are necessarily affected by the corresponding acts. Those that were not there before, spring up: the rest gain in strength and extent.
(LXXV, The Golden Sayings of Epictetus, trans. by Hastings Crossley, 1909)
Often, the rationalization given at a moment is that one will do it just the once. Epictetus is exactly right. “Do not reckon it a single defeat.” If one can impress that upon one’s mind, it should strengthen one’s resolve – more is at stake than an isolated act.
The flip side to this state of affairs is that doing something that would lead to a good habit also affects the corresponding habits and faculties.