One line and two verdicts. γελοῖος and ξένος — ridiculous, and a stranger. The second word matters more than the first and usually drops out in translation: astonishment is declared not an error of judgement but a mark of not being from here. That is how a visitor behaves, for whom the customs of the country are news; and Marcus says that a man astonished at the course of things is a visitor in his own world.

Where the measure comes from. The passage stands in direct connection with 12.1, where it is promised that a man living according to nature "will cease to be a ξένος in his own native land and to wonder at the daily as though it were unlooked for" (θαυμάζων ὡς ἀπροσδόκητα). 12.13 is that same phrase compressed to a single line and turned into a diagnosis. Cf. 4.29: a stranger twice over is he who knows neither what is in the cosmos nor what happens in it.

ὁτιοῦν. "Anything whatever" — an intensifier, and it closes the loophole: the subject is not great calamities but any event at all. The measure taken is not endurance but knowledge: a man who knows the nature of things has nothing to be astonished at, just as no one is astonished that a fig tree yields a fig (8.15, 4.6 — where the verb is ξενίζεσθαι, from the same root as ξένος: to be astonished, in Greek, is to take a thing for foreign).

The genre. No command and no argument — a nominal exclamation, like 12.7 and 12.10. Book XII increasingly reduces its entries to a single formula: the work is already done, and what remains is the tag by which it can be recovered. The governing discipline is assent, the sustaining one desire.