Prohairesis literally means "preferential choice" (from προ- + αἱρέω, "to choose in advance"). It is an Aristotelian technical term, introduced in Nicomachean Ethics III 2–4: the deliberate choice of means after deliberation upon the end. In Aristotle, prohairesis is rational wanting — the intersection of βούλευσις (deliberation) and ὄρεξις (desire): one chooses what one has deliberated about.

In Epictetus, prohairesis undergoes a radical transformation and becomes the central technical term of his whole system. Epictetus redefines prohairesis as the faculty of choosing itself — not a single act but a continuous inner instance that localises moral freedom. In him prohairesis:

• is the only thing that is genuinely "ours" (τὰ ἐφ' ἡμῖν, see dichotomy-of-control);
• cannot be coerced by any external force;
• is identical with the person in his moral dimension — Epictetus says: "you are your prohairesis, not your body, your possessions, your reputation" (Disc. 3.1.40);
• is the source of freedom (ἐλευθερία) and the seat of αἰδώς (αἰδώς is "the daughter of prohairesis," Disc. 1.28.20).

In Stoic orthodoxy (Zeno, Chrysippus), prohairesis is not the central term — there the central concepts are TERMσυγκατάθεσις (assent) and TERMἡγεμονικόν (the ruling part). Epictetus introduces prohairesis as the functional aspect of ἡγεμονικόν — the same ruling part viewed from the side of its electing activity.

Aristot. Eth. Nic. III 2–4, 1111b4–1113a14 (the classical statement); VI 2, 1139a31–b5 (διανοητικὴ ὄρεξις, "deliberative desire"); SVF II 988 (Stoic usage, not yet central); Epict. Disc. 1.1 ("what is up to us"); 1.4 (on prohairesis as the locus of προκοπή, moral progress); 1.17.21–28; 2.10.1; 2.23.16; 3.1.40 ("you are not this body, you are prohairesis"); 4.1.99–100; 4.5.12 ("you are not flesh or hair, but prohairesis"); Ench. 1, 9, 13, 30; LS 62.

In Marcus the explicit term prohairesis occurs very rarely — he works with the same concept under the names TERMἡγεμονικόν, ὁ ἔνδον δαίμων, and νοῦς. The historical reason: Epictetus died c. 135 CE, Marcus was born in 121, and read Epictetus through Junius Rusticus — so Marcus knew Epictetus' terminological innovation but preferred, in his Meditations, the "classical" Stoic vocabulary (ἡγεμονικόν) under the influence of Chrysippus. Conceptually, however, Marcus' "ruling part" is functionally identical with Epictetus' prohairesis.

Epictetus' prohairesis is a significant contribution to the history of Western thought: the first such explicit localisation of moral freedom in an inner instance inaccessible to any external force. The concept feeds directly into Augustine (free will as the seat of choice), into Kant (autonomy), and in the twentieth century into the existentialists (choice as the essential determination of being). Cf. also the doctrine self-is-hegemonikon — the Stoic version of the same intuition in the vocabulary of ἡγεμονικόν.