Eurhoia biou — "the smooth flow of life" — is the Stoic technical definition of TERMhappiness (εὐδαιμονία). It goes back to Zeno: τέλος εἶναι τὸ ὁμολογουμένως ζῆν, ὅπερ ἐστὶ καθ' ἕνα λόγον καὶ σύμφωνον ζῆν — "the end is to live in agreement, that is, to live by one reason, in concord." And such a life is εὔρους βίος — a flow of existence without internal discordances, like a river that runs evenly, without rapids or jams. Not a "pleasant" life and not a "successful" one, but a life internally consistent: reason, desire, and action move in one direction, with no resistance to themselves.

SVF I 184 (Zeno's τέλος-formula); III 4–9 (Chrysippus' development); the classical definition: Stob. Ecl. II 77 W: "εὐδαιμονία δ' ἐστὶν εὔροια βίου" ("happiness is the smooth flow of life"). DL VII 88; Cic. De fin. III 26; LS 63. The form εὔρους βίος appears in Marcus precisely at 02-05; the cognate idea at Med. 05-09; 05-34; 10.6 (the flow of nature); 12.27.

In 02-05 εὔρους βίος appears in the culminating phrase: "see how few things one must master in order to live a εὔρουν καὶ θεουδῆ βίον — a smoothly-flowing, god-fearing life." The metaphor of the stream works at two levels: (a) inwardly — the smooth agreement of reason with itself, without jolts, passions, or contradictions; (b) cosmically — agreement with the stream of the cosmos, that is, with fate and providence. Stoic eurhoia is not the absence of movement but its right mode. See also the discipline of desire and the doctrine of the self-sufficiency of virtue: eurhoia requires only the correct disposition of reason, nothing "external."