The book ends with a scene, not a conclusion. The last passage of Book XI is neither a formula nor an exhortation but a cross-examination in four moves1, written down without a word of Marcus's own. There is no marking of the speeches and no name for the interlocutor: the dialogue runs on dashes alone, and the speakers must be identified from the sense.
How the trap is built. The first two questions are harmless and win assent: rational, not irrational; sound, not worthless. The third — τί οὖν οὐ ζητεῖτε; "why then do you not look for them?" — is already the catch, and the answer is supplied by the interlocutors themselves, unawares: ὅτι ἔχομεν, "because we have them." The fourth question adds nothing; it merely collides that answer with what can be seen: τί οὖν μάχεσθε καὶ διαφέρεσθε; "why then do you fight and quarrel?"
What has been proved. Nothing — and there lies the force. The conclusion is not stated: if you are fighting, then either your soul is not sound or you do not think it sound. Both limbs are fatal to a man who has just said "we have them." Cf. 11.38: the contest is not over a trifle but over being mad or not — and 11.18, given over entirely to how not to quarrel.
Two words on what matters. ὑγιῶν ἢ φαύλων — "sound or worthless": a pair at once medical and evaluative, as everywhere in the corpus (cf. 10.35, where a mind is judged diseased by the single mark of fastidiousness). And ἔχομεν, "we have": the most dangerous word in the dialogue, because possession is taken here as a given, whereas 11.1 says that the rational soul makes itself whatever it wishes. To have it sound is not a state but a labour. The governing discipline is assent, the sustaining one action.