At the beginning of the Enchiridion, Epictetus notes the things that are in our power, or ‘up to us,’ and the benefits of keeping these as our focus:
Of things some are in our power, and others are not. In our power are opinion, movement toward a thing, desire, aversion (turning from a thing); and in a word, whatever are our own acts: not in our power are the body, property, reputation, offices (magisterial power), and in a word, whatever are not our own acts. And the things in our power are by nature free, not subject to restraint nor hindrance: but the things not in our power are weak, slavish, subject to restraint, in the power of others. Remember then that if you think the things which are by nature slavish to be free, and the things which are in the power of others to be your own, you will be hindered, you will lament, you will be disturbed, you will blame both gods and men: but if you think that only which is your own to be your own, and if you think that what is another’s, as it really is, belongs to another, no man will ever compel you, no man will hinder you, you will never blame any man, you will accuse no man, you will do nothing involuntarily (against your will), no man will harm you, you will have no enemy, for you will not suffer any harm.
Enchiridion, translated by George Long, Prometheus Books.