Conservation of momentum. Marcus Aurelius 5.13

Marcus witnesses that he is part of the universe, a momentary effect of past causes that becomes itself cause for future effects, whose birth will eventually entail its own unmaking. This is mortality, for us, but we cannot see the mortality of the universe as clearly as we see our own: to us, the universe appears as an endless succession of causes and effects, worlds without end.

Ἐξ αἰτιώδους καὶ ὑλικοῦ συνέστηκα, οὐδέτερον δὲ τούτων εἰς τὸ μὴ ὂν φθαρήσεται, ὥσπερ οὐδὲ ἐκ τοῦ μὴ ὄντος ὑπέστη. οὐκοῦν καταταχθήσεται πᾶν μέρος ἐμὸν κατὰ μεταβολὴν εἰς μέρος τι τοῦ κόσμου καὶ πάλιν ἐκεῖνο εἰς ἕτερον μέρος τι τοῦ κόσμου μεταβαλεῖ καὶ ἤδη εἰς ἄπειρον. κατὰ τοιαύτην δὲ μεταβολὴν κἀγὼ ὑπέστην καὶ οἱ ἐμὲ γεννήσαντες καὶ ἐπανιόντι εἰς ἄλλο ἄπειρον. οὐδὲν γὰρ κωλύει οὕτως φάναι, κἂν κατὰ περιόδους πεπερασμένας ὁ κόσμος διοικῆται.

I am a creature made of form and matter, cause and effect. Neither of these twain will decompose into nothing, just as neither of them takes its existence from nothing. Thus it follows that every part of me shall in time be digested and become part of the universe, wherein it shall afterward change its station according to a process of alteration without end. In keeping with this process, my ancestors and I exist, and submit always to the one who comes after us—the endless effect of our cause. Nothing impedes us from remarking that the world is arranged thus, in repeating cycles that know and keep their bounds.