What does Marcus Aurelius say actually disturbs us, if not other people's acts?

Marcus writes it plainly: "it is not men's acts which disturb us, for those acts have their foundation in men's ruling principles, but it is our own opinions which disturb us" (Meditations 11.18). The acts themselves are outputs of another person's ruling principle, their capacity for judgment and choice, which belongs to them and not to you. What actually lands on you, what produces the flash of anger or the ache of grief, is a judgment you have made about that act: that it is grievous, that it should not have happened, that it is a personal injury.

The common misreading treats this as a claim about perception versus reality, as if Marcus is saying the wrong done to you is not really so bad, or that you should talk yourself into thinking it does not matter. That is not the argument. He is making a claim about causation. Two people can watch the same act, a colleague's betrayal, a stranger's rudeness, and one is consumed by it while the other is barely touched. The act was identical. What differed was the opinion each person attached to it. So the opinion, not the act, is doing the disturbing.

This matters because it tells you where the lever is. You cannot reach into another person's ruling principle and edit it. You have no power over why they did what they did or whether they will do it again. But you do have power over your own judgment about what happened, because that judgment is yours, formed by you, sustained by you. Marcus draws the practical conclusion directly: "Take away these opinions then, and resolve to dismiss your judgement about an act as if it were something grievous, and your anger is gone" (Meditations 11.18). Remove the opinion that this is a grievous thing, and the anger has nothing left to stand on.

He anticipates the obvious question, how do I actually take away the opinion, and gives one route: reflect that no wrongful act of another brings shame on you, since if shame came automatically from proximity to wrongdoing you would be shamed constantly by the many bad acts you have witnessed in your life (Meditations 11.18). The wrong is theirs. Your character is not implicated in it unless you make it so through your own reaction.

He also makes a sharper, almost clinical observation about cost. Consider, he says, how much more pain the anger and vexation cause you than the original act ever did (Meditations 11.18). The wound from the act is usually smaller and shorter than the wound from your own resentment about it. You get angry at the insult, then you replay it for a week. The replaying is the larger injury, and it is entirely self-inflicted.

None of this asks you to pretend the act was fine, or to suppress the sting of a first reaction. Marcus elsewhere treats things like pain and dishonor as neither good nor evil in themselves, since they happen indiscriminately to good and bad people alike (Meditations 2.11). That is a separate claim about value. Here the claim is narrower and more useful day to day: your suffering tracks your judgment, not the raw event. So when you catch yourself furious at what someone did, the honest next question is not what should they have done differently, but what am I telling myself this means. That is the opinion you have the power to examine, and the only one you were ever going to be able to change.