Marcus Aurelius, answered from the texts and cited to the page.
The ruling faculty — the hegemonikon — when it gathers itself and consents only to what it chooses, is invincible.1 That is the citadel. Not a wall you build around yourself, not a retreat from the world, but the discovery that no external thing can compel your judgment without your permission. What disturbs you is never the thing itself. It is your verdict about the thing.2
Wipe out the verdict and the disturbance goes with it. This is not a consolation; it is a precise description of where in the causal order you actually live. The event reaches the body, the senses, the reputation — none of that is yours to keep. The assent is yours entirely. The mind free from passions is the citadel.3 He who has not seen this is simply ignorant of where his strength lies.
He who has seen it and still does not take refuge there — that man is the one I pity, because he has the fortress and sleeps outside its walls. One practical discipline follows from this. When an impression arrives, hold it at its first appearance and add nothing.4 Someone has spoken ill of you: that much has arrived. That you have been harmed — that has not arrived; you brought it with you.
A child is sick: visible. That the child is in danger: not yet given. Stay with what is actually before you and stop there. Every addition beyond the raw appearance is your own manufacture, and it is in that addition that suffering lives.
Remember that the ruling faculty is invincible, when self-collected it is satisfied with itself, if it does nothing which it does not choose to do, even if it resist from mere obstinacy.Meditations
If thou art pained by any external thing, it is not this thing that disturbs thee, but thy own judgment about it. And it is in thy power to wipe out this judgment now.Meditations
Therefore the mind which is free from passions is a citadel, for man has nothing more secure to which he can fly for refuge and for the future be inexpugnable.Meditations
Say nothing more to thyself than what the first appearances report... thus then always abide by the first appearances, and add nothing thyself from within, and then nothing happens to thee.Meditations