A quotation that does not suffice. The passage opens with someone else's formula in quotation marks1 and immediately limits it: οὐκ ἀρκεῖ τὸ εἰρημένον — "what has been said is not enough." This is a rare genre in Marcus: neither assent nor dispute, but further specification. The borrowed thought is true but empty: it demands a single aim without saying which.
Self-identity as a function of the aim. MOTIFὯι μὴ MOTIFεἷς καὶ ὁ αὐτόςMOTIF … σκοπός, οὗτος MOTIFεἷς καὶ ὁ αὐτὸςMOTIF … εἶναι οὐ δύναται — one and the same formula is spoken twice: first of the aim, then of the person. The unity of a personality is not given but derivative: as many aims, so many selves. Hence the link with MOTIFaimless wandering — in Marcus this is not absent-mindedness but a coming-apart into several "I"s.
An argument by analogy with opinion about the good. The analogy is built on TERMὑπόληψις: not every opinion about what the majority take to be TERMgood is uniform — the common ones (τὰ κοινά) are singled out from the mass. So too with the aim: not any will do, but one of the same kind. The logic is this: the aim must belong to the class about which non-conflicting agreement is possible — and only the common interest is such.
The conclusion in one word. τὸν σκοπὸν … τὸν TERMκοινωνικὸν DOGMAκαὶ πολιτικὸν DOGMAὑποστήσασθαι — "the aim must be set underneath oneself" (ὑφίστημι, "to lay as a foundation"). Not chosen, nor set before one, but laid under everything as a footing. And the mechanism: ὁ … πάσας τὰς ἰδίας TERMὁρμὰς ἀπευθύνων — "straightening all his own private impulses toward it" (ἀπευθύνω, to true up along a line, as a beam is trued). The result: πάσας τὰς πράξεις ὁμοίας ἀποδώσει — all his acts will come out of one kind, and thereby ἀεὶ ὁ αὐτὸς ἔσται.
Why this is the discipline of action. What is at issue is not how to judge what happens but what the whole body of one's acts is fastened to. Cf. 7.4 (at every impulse, to what mark is it referred) and 9.23 (an act with no reference to the common end "tears apart" one's life and proves seditious). The governing discipline is action, the sustaining one assent.