It is one of the shortest entries in the Meditations. In George Long's translation of the sixth book, the sixth note reads in full: "The best way of avenging thyself is not to become like the wrong doer."
The note before it is about reason knowing its own disposition. The note after it is about taking pleasure in passing from one social act to another. Between them sits a single sentence about revenge, written by the most powerful man in the world to nobody but himself.
That context matters for reading it. Marcus Aurelius was emperor of Rome and could have avenged himself on anyone he liked. The Meditations are not a public teaching. They are a notebook of things he needed to tell himself, and this line is one of them. It is not advice to the weak about making peace with being wronged. It is a reminder from a man with every means of retaliation that retaliation was a way of losing.
The claim is perhaps stronger than it first looks. Marcus is not saying that revenge is wrong, or beneath you, or necessarily unproductive. He is saying something about what revenge does to the avenger: that in repaying an injury in kind you take on the character of the person who injured you, and that this is the real loss, larger than the original harm.
What revenge does to the avenger
Marcus probably had no way to scientifically test whether avenging yourself makes you worse off. In 2008, three psychologists did. Kevin Carlsmith, Timothy Wilson and Daniel Gilbert, ran an experiment described in a 2011 cover story in the Observer, the magazine of the Association for Psychological Science.
Here's how it went. Participants played an investment game in groups of four. Each person got a dollar to keep or put into a shared pot, which was then increased and divided evenly. One member of each group was secretly working for the researchers, and kept their dollar while taking a share of everyone else's: a free rider.
Then, some participants were given the power to punish the free rider, at a small cost to themselves, by cutting the free rider's earnings. Others learned what had happened but could do nothing about it. A third group was asked only to predict how they would feel if they could punish. Everyone rated their mood immediately and again ten minutes later.
The people who predicted said revenge would make them feel better. The people who actually took it felt worse: worse than the forecasters had predicted, and worse than the people who had been given no chance to act at all. Ten minutes on, the punishers were still thinking about the free rider more than the non-punishers were.
"I think uncertainty prolongs and enhances emotional experiences," Carlsmith said, and "one of the things that avengers do unintentionally is to prolong the unpleasant encounter." The people who could not retaliate had no such option. "Those who don't have a chance to take revenge are forced, in a sense, to move on and focus on something different. And they feel happier."
The exchange refused
None of this makes the sentence easy to live by. Being wronged produces a want to answer in kind that no notebook entry dissolves, and the Stoic answer, that the want is itself the thing to master, is a high standard. The Meditations are full of reminders because the reminders kept being needed.
Avenging yourself keeps the wrongdoer in your head, and the longer they stay there the more of your day they own. Marcus's phrase for the alternative is to refuse to become like them.
The people in the study who were denied revenge did not choose that restraint, but they got its benefit anyway: ten minutes later, they were thinking about it less. The ones who got their revenge were still, in every sense that mattered, in the room with the person who had cheated them.