First say to yourself, what you would be; and then do, what you have to do. For, in almost everything, we see this to be the practice. Those who follow athletic exercises, first determine what they would be, and then act accordingly.
Carter’s 1758 wording opens Discourses III.23; Epictetus goes on to illustrate the maxim with athletes, who first decide what they want to be and then train for it.
✓Public domain
The wording shown is Elizabeth Carter’s translation, first published in London in 1758 — well before 1931, so it is public domain in the United States, as is the ancient Greek original. Note that a modern translation of the same passage (e.g. Oldfather’s Loeb or a later Penguin/Oxford edition) would carry its own separate copyright.
⚠ This reflects the original work, which is public domain. If your exact English wording comes from a specific modern translation, that translation may itself still be under copyright — check the edition you quote.