Marcus Aurelius, answered from the texts and cited to the page.
What is in your power, and what is not — this is the whole of it. Epictetus, from whom I learned more than from any other teacher, put the matter plainly: no man can rob us of our free will.1 That is the positive side of the dichotomy. The negative side is equally precise: with respect to avoidance, he said, a man should not show it with respect to any of the things which are not in our power.2
Together these two moves carve the world at its joint — what belongs to you, and what does not. What belongs to you is the hegemonikon, the ruling faculty, the seat of judgment and assent. What does not belong to you is everything outside it: the body, reputation, the opinions of others, the hour of death, the outcome of your actions. The discipline is not merely intellectual.
It requires an art of assent — specific, practiced, applied to each impression as it arrives.2 I put it to myself this way in Book XII: neglect everything else and respect only the ruling faculty and the divinity within you.3 Not because the outside world is nothing, but because demanding that it conform to your preference is the one guaranteed path to slavery.
Direct the present conformably to piety and justice; trust the future to providence; take no notice of the past.3 What remains after those three moves is entirely yours — and it was always entirely yours. The dichotomy does not shrink your life. It shows you where your life actually is.
No man can rob us of our free will (Epictetus, iii. 22, 105).Meditations
in respect to his movements he must be careful that they be made with regard to circumstances...and as to avoidance [aversion], he should not show it with respect to any of the things which are not in our power.Meditations
If, then, whatever the time may be when thou shalt be near to thy departure, neglecting everything else thou shalt respect only thy ruling faculty and the divinity within thee...then thou wilt be a man worthy of the universe which has produced thee.Meditations