Place in the book. The fifth passage is about character as the domain of "what is up to us." Marcus separates a natural gift (a quick mind) from the moral qualities open to everyone, and cuts away the chief excuse of sloth: "I was not born for it."
The gift and what is up to us. Sharpness of wit (δριμύτης) is an endowment, not a merit; nature has not given it to all. But there are "many other things" of which one cannot say «οὐ γὰρ πέφυκα» ("I am not formed for it") — for they are DOGMAaltogether in our power (ἅπερ ὅλα ἐστὶν ἐπὶ σοί). This is a pure locus of the dichotomy of control: do not confuse the not-up-to-us (talent) with the up-to-us (character).
The catalogue of available virtues. Eleven qualities, all of them Marcus's characteristic negative-compound coinages: ἀκίβδηλον (unadulterated, "sincerity"), σεμνόν (dignity), φερέπονον (endurance), ἀφιλήδονον (no fondness for pleasure), ἀμεμψίμοιρον (no complaint at one's lot), ὀλιγοδεές (wanting little), εὐμενές (benevolence), ἐλεύθερον (freedom), ἀπέρισσον (without excess), ἀφλύαρον (without chatter), μεγαλεῖον (greatness of soul). None requires talent — all are available now.
"Voluntarily below the mark." The diagnosis is sharp: you remain at your former level ἑκών — "by your own will." And the vices — to murmur, be grasping, flatter, blame the "poor little body" (σωμάτιον), fawn, show off, be tossed about in TERMsoul — are likewise not forced by any natural incapacity (ἀφυΐα). With the oath "no, by the gods," Marcus rejects the fatalist's excuse: you could have been rid of all this long ago.
Even slowness is a matter for training. The final move is subtler: even if you really are "slow of comprehension" (βραδύτερος), this is no verdict — "this too must be trained" (ἀσκητέον), without indulging laziness and "without taking pleasure in the sluggishness" (ἐμφιληδονοῦντι τῇ νωθείᾳ). Character is not a gift but an ἄσκησις; hence the anti-fatalist edge of the whole passage.
Discipline and parallels. The principal discipline is action (the forming of character); the secondary is desire (contentment with one's lot, wanting little, indifference to pleasure). Parallels: the dichotomy — Epictetus Ench. 1; "do not discuss what a good man is — be one" — Med. 10.16.