Definition: The idea that some things are within our control, and others are not.
Reference: Epictetus, Enchiridion: "Some things are within our power, while others are not. Within our power are opinion, motivation, desire, aversion, and, in a word, whatever is of our own doing; not within our power are our body, our property, reputation, office, and, in a word, whatever is not of our own doing."
