Marcus Aurelius, answered from the texts and cited to the page.
The ruling faculty — the hegemonikon — is the seat of everything that matters.1 What strikes it from outside is an impression: a report that someone has spoken ill of you, that your child is sick, that an obstacle has closed off what you intended to do. The impression arrives; it cannot be helped. What you do next is entirely yours.2 Assent is that next move.
To assent is to add your judgment to the bare impression — to say not merely this has been reported but this is an injury, not merely I see the child is sick but he is in danger. The discipline is to hold at the first appearance and add nothing.3 Pain, fear, grief: these do not come from the thing itself but from the judgment you lay over it.4
Wipe out the judgment, and you wipe out the suffering — not by ceasing to see, but by refusing to see more than what is actually there. The mind free from passions is a citadel.5 Nothing external can compel it. Even from sheer obstinacy, the ruling faculty resists what it does not choose; how much more when it acts with reason and deliberate judgment.6
The hegemonikon, self-collected and satisfied with itself, is invincible — and the man who has seen this and does not take refuge there is, of all men, the most pitiable. Separate from your understanding whatever others do or say, whatever you have done or said, whatever future things trouble you because they may happen — and the intellectual power, exempt from the things of fate, lives pure and free, doing what is just, accepting what comes, saying the truth.7 That is the whole of it. The impressions will keep arriving. The discipline is what you do at the threshold.
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.Meditations
Say nothing more to thyself than what the first appearances report... This has been reported; but that thou hast been injured, that has not been reported.Meditations
Thus then always abide by the first appearances, and add nothing thyself from within, and then nothing happens to thee.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
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
What then will it be when it forms a judgment about anything aided by reason and deliberately?Meditations
if thou shalt separate from thyself, that is, from thy understanding, whatever others do or say, and whatever thou hast done or said thyself, and whatever future things trouble thee because they may happen... so that the intellectual power exempt from the things of fate can live pure and free by itself, doing what is just and accepting what happens and saying the truth.Meditations