Prohairesis, choice or volition, is for Epictetus the self in its essential capacity to assent, desire and choose. Nothing outside can compel it; even Zeus, he says, cannot overpower it. All of Stoic training aims to keep the prohairesis in accord with nature.
Prohairesis
Moral choice: in Epictetus, the faculty of will and judgement that alone is truly our own.
Also: volition