Daimōn (δαίμων) is one of the oldest Greek religious words, with a rich evolution of meanings:

• Homeric sense — a divine power in general, often personified as "lot" or "portion" (from δαίω, "to distribute"); may designate any god or divine agency.
• Hesiod (Op. 122–126) — the people of the Golden Age, after death, became δαίμονες, guardians of the earth. From this grows the classical sense of "guardian spirit" or "guiding genius."
• The Socratic daimonion (Plat. Apol. 31c–d; Phaedr. 242b) — an inner cautionary voice. Socrates describes it as "something divine and δαιμόνιον" that sometimes tells him "no," never "yes."
• Plato (Resp. X 617e: "the daimōn does not choose you — you choose your daimōn"; Tim. 90a–c) — the daimōn is the highest rational part of the human soul, through which a person makes contact with the divine.
• The Stoics (via Epictetus, Disc. 1.14.12–14) — to every person god has given his own δαίμων as overseer, and this δαίμων is his rational part, his TERMruling part.

In Stoic technical terminology, δαίμων is functionally identical with TERMἡγεμονικόν, TERMνοῦς, and TERMθείῃ ἀπομοίρᾳ: it is the same rational and divine part of the soul, viewed under a particular aspect. Their nuances differ:

• ἡγεμονικόν foregrounds the governing function (it rules the soul);
• νοῦς foregrounds the noetic function (it sees truth);
• θεία ἀπομοῖρα foregrounds ontological origin (a portion broken off from god);
• δαίμων foregrounds the personal-guiding character (my own guardian spirit).

Hom. Il. 1.222 and passim (the Homeric δαίμων); Hes. Op. 122–126; Plat. Apol. 31c–d (the Socratic δαιμόνιον); Phaedr. 242b; Resp. X 617e; Tim. 90a–c (the daimōn as the highest part of the soul); SVF I 538 (Cleanthes, on the daimōn and the Logos); Epict. Disc. 1.14.12–14; 2.8.10–14 ("you carry god within you and do not know it"); LS 47, 53. In Marcus, δαίμων is a frequent term: Med. 2.13; 2.17; 3.4; 3.5; 3.6; 3.7; 3.16; 5.10; 5.27; 7.17; 8.45; 12.3; 12.26.

In 02-13, δαίμων makes its first appearance in Book II with a direct definition: ὁ ἔνδον δαίμων — "the daimōn within." Marcus prescribes "serving it sincerely" (γνησίως θεραπεύειν), and this service (θεραπεία) reduces to three negative operations: to keep it (a) pure from passion (καθαρὸν πάθους), (b) from rashness (εἰκαιότητος), and (c) from discontent with the works of gods and men (δυσαρεστήσεως).

In Hadot's reading, this triad corresponds to the three disciplines:

• "pure from passion" → discipline of desire (mastery of emotions)
• "from rashness" → discipline of assent (precise judgment)
• "from discontent with what is" → discipline of desire / action (acceptance of fate)

The etymological tie with TERMεὐδαιμονία (= εὖ-δαίμων-ία, "the condition of having a good daimōn") here gains practical substance: εὐδαιμονία is not "happiness in general" but specifically a life lived in the right relation to one's own δαίμων. To live εὖ with one's δαίμων is to serve it purely — without passion, without distraction, without revolt against the real. This is the anti-etymology of κακοδαιμονία: a person lives badly who lets his daimōn fall into neglect or subjects it to passion. Compare Med. 3.16: "the bodily — rivers; the psychic — dreams and vapor; life — war and a stranger's sojourn… what can guide [a person]? One thing only — philosophy. And philosophy lies in keeping the daimōn within unoffended."

Foucault, in L'herméneutique du sujet (lecture of 12 January 1982), observes that Marcus's formulation ἔνδον δαίμων marks a turning point in the ancient "care of the self" (ἐπιμέλεια ἑαυτοῦ): here, what is cared for has a genuinely inner character, and the care itself takes the structure of service. It is a step toward late-antique and Christian interiority — long before Augustine.