A single sentence, and with no verb of command. Neither "remember" nor "do" — only an exclamation about what strikes the eye of itself: Πῶς ἐναργὲς προσπίπτει — "how plainly it falls upon one" (προσπίπτω, "to fall toward," the verb Marcus uses of the arrival of an impression; cf. 10.31). The thought is offered not as the conclusion of an argument but as an immediate evidence one has only to avoid overlooking.

The key word is ὑπόθεσις. βίου ὑπόθεσις is not "conditions of life" in general but "what has been laid under you," what is set beneath you as an assignment: the plot, the circumstances, the part. It is exactly the word of 10.31: Οἵαν ὕλην καὶ ὑπόθεσιν φεύγεις — "what material and what assignment you are running from!" There it is said in reproach, here in wonder; the operation is the same: what looks like an obstacle to philosophy is named its condition.

Who is saying this, and about what. The force rests on what is left unsaid: the writer is an emperor in a military camp, amid business, retinue and war — that is, in the situation anyone would call the least fit for philosophy. Hence the weight of ἐπιτήδειον ("suited, ready to hand"): not "tolerable" but better than any other. Philosophizing requires neither leisure nor withdrawal; cf. 4.3 (no need of seashore or hills) and 10.15 (a mountain is not a place but a mode of presence).

Why this is the discipline of desire. The passage does not advise resignation to one's circumstances — it cancels the very thought that other ones are needed. The wish for another place, another time, another office is removed not by prohibition but by devaluing its object: there simply is no better. The governing discipline is desire, the sustaining one action.