Genre and place in the book. A famous ethical maxim — and a turn from the physics-theology of Book VI's opening (06-01…06-05) to dealing with people. A single line on how to answer an injury: the best "revenge" is not requital in kind but the refusal to become like the offender. The principal discipline is action (one's stance toward the wrongdoer); the secondary is assent (guarding one's own ruling part from corruption).

To requite (ἀμύνεσθαι). The verb ἀμύνεσθαι spans both "to defend oneself" and "to requite, to avenge" — hence Long's "avenging" and Rogovin's "to defend oneself." Marcus plays on the ambiguity: the true "requital" of an offender is not a counter-blow but the keeping of oneself. Thus "revenge" is redefined — not to do something to him, but to deny him the power to make you what he is.

Not to become like (μὴ ἐξομοιοῦσθαι). If you answer injury with injury, anger with anger, you take on the offender's vice, you "become like" him; and that alone is the real harm — but one inflicted on yourself. Behind the maxim stands the doctrine that the virtuous cannot be harmed: an external injury cannot touch your good "unless it makes your ruling faculty worse than it was" (Med. 7.22). And what makes it worse is not another's injustice but your own consent to answer in kind.

The therapy of anger. The obverse is Med. 9.4: "he who acts unjustly acts unjustly to himself, because he makes himself bad." The offender has already punished himself — he has become worse; it remains only for you not to follow him. The ground of anger thus falls away: there is nothing to resent, for he cannot harm you, and he has already harmed himself. Gentleness here is not weakness but at once self-preservation and the surest "requital."

Disciplines and parallels. The principal one is action (how to answer a wrongdoer); the secondary is assent (not letting the ruling part become like his). Parallels: Med. 9.4 (the wrongdoer harms himself); 7.22 (it is peculiar to man to love even those who do wrong; he has not made your ruling faculty worse); 02-01 (human beings are made for one another; the erring sin through ignorance of good and evil). The doctrine — no harm can come to the virtuous; cf. Plat. Apol. 30c–d ("the worse cannot harm the better"); Sen. De const. sap.