Eudaimonia is the central concept of Greek ethics: the life worth living; the "successful" life taken as a whole. Etymologically εὖ + δαίμων: "in the company of a good daimōn," that is, with a favourable inner guardian (or rather inner divine principle — which for the Stoics is the ruling part, identical with the divine portion). Each school gives eudaimonia its own specification: for Epicurus — ἀταραξία (the absence of mental disturbance); for Aristotle — ἐνέργεια κατ' ἀρετήν (activity in accordance with virtue, sustained by a complete life with external goods); for the Stoics — TERMεὔρους βίος (the smooth flow of life), achieved through DOGMAvirtue alone.

The morphological antonym is TERMκακοδαιμονία (κακο- + δαίμων), which has its own card. In Marcus the verbs εὐδαιμονεῖν and κακοδαιμονεῖν work as a binary opposition: there is no third option — either you live εὖ with your daimōn-as-intellect, or you live κακῶς. Since eudaimonia and kakodaimonia are distinct concepts (not one concept in different forms), they are treated in separate cards; the cross-references sit here and in TERMkakodaimonia.

Aristot. Eth. Nic. I 4–13 (the classical statement of the question); SVF I 184 (Zeno's formula — eudaimonia as εὔροια βίου); III 16, 49–67 (Chrysippus); DL VII 87–89; Stob. Ecl. II 77 W (the classical Stoic definition). The term eudaimonia itself is rare in Marcus; its specific Stoic forms — εὔρους βίος (02-05) and εὐμοιρία (02-06) — appear more often; the term itself, with its etymology unpacked, at 07-17 (εὐδαιμονία = δαίμων ἀγαθός).

In 02-06 Marcus uses the rare form εὐμοιρία — literally "a good TERMμοῖρα," that is, "a good lot," "a well-drawn share." It is synonymous with eudaimonia but semantically foregrounds what is measured out, apportioned, received by lot. The cluster is internally coherent: τὰ συμμεμοιραμένα in 02-05 — "what is measured out together with the whole" (also from the root TERMμοῖρα) — and εὐμοιρία in 02-06 — the good state of that measured portion. Hence the paradox of self-reproach: the soul places its εὐμοιρία in other souls, whereas by definition εὐμοιρία is the state of my own TERMμοῖρα and admits of no placement outside myself. See TERMheimarmene (the cosmological "measuredness" of all that is), dichotomy-of-control (the structural cause of the error), and aidos (the inner correlate of a properly drawn boundary).