Of all existing things some are in our power, and others are not in our power. In our power are thought, impulse, will to get and will to avoid, and, in a word, everything which is our own doing. Things not in our power include the body, property, reputation, office, and, in a word, everything which is not our own doing. Things in our power are by nature free, unhindered, untrammelled; things not in our power are weak, servile, subject to hindrance, dependent on others. Remember then that if you imagine that what is naturally slavish is free, and what is naturally another's is your own, you will be hampered, you will mourn, you will be put to confusion, you will blame gods and men; but if you think that only your own belongs to you, and that what is another's is indeed another's, no one will ever put compulsion or hindrance on you, you will blame none, you will accuse none, you will do nothing against your will, no one will harm you, you will have no enemy, for no harm can touch you.
Aiming then at these high matters, you must remember that to attain them requires more than ordinary effort; you will have to give up some things entirely, and put off others for the moment. And if you would have these also - office and wealth - it may be that you will fail to get them, just because your desire is set on the former, and you will certainly fail to attain those things which alone bring freedom and happiness.
Make it your study then to confront every harsh impression with the words, 'You are but an impression, and not at all what you seem to be'. Then test it by those rules that you possess; and first by this - the chief test of all - 'Is it concerned with what is in our power or with what is not in our power?' And if it is concerned with what is not in power, be ready with the answer that it is nothing to you. (1, p. 213)
Epictetus. The Discourses and Manual. Trans. P.E. Matheson. Oxford University Press 1916.
Works and Days