Thymos is one of the most capacious words of Greek psychology, with a long pre-philosophical history. In Homer it is vital force, breath-soul, the seat of the passions; in Plato (Resp. IV 439e–441c) it is the "spirited" element of the soul (θυμοειδές), intermediate between the rational (λογιστικόν) and the appetitive (ἐπιθυμητικόν), whose function is to uphold honour and virtue in the struggle against lower drives. In Aristotle (EN VII), thymos is a strong movement toward revenge for a wrong suffered.

The Stoics rejected Plato's tripartite model of the soul: for them the soul is one; in a rational being everything psychic is the TERMruling part, and the soul's movements are its assents. In the Stoic taxonomy of the passions, θυμός is not an independent cardinal passion but a species of ὀργή (anger), itself a species of TERMἐπιθυμία as "the desire for retaliation" (ὄρεξις τιμωρίας) for an apparent wrong done. Chrysippus gave it even a special definition: θυμός = ὀργὴ ἀρχομένη — "anger at the beginning of its development," the first, still-hot phase before passing into settled resentment (μῆνις) or designed revenge (κότος).

Hom. Il. passim (the Homeric θυμός); Plat. Resp. IV 439e–441c (the tripartite division of the soul); Aristot. EN VII; Rhet. II 2 (ὀργή); SVF III 394–397, 411–414 (the Stoic classification of θυμός as a species of ὀργή / ἐπιθυμία); DL VII 114; Stob. Ecl. II 91 W; LS 65. In Marcus: Med. 2.10; 9.42; 11.18 (the full "therapy of anger" in nine points); 12.27.

In 02-10 Marcus works within a Theophrastean (not strictly Stoic) frame: ἐπιθυμία and θυμός are opposed as parallel categories, where in the pure Stoic taxonomy the second is subordinated to the first. This is typical of Marcus: he retains the Peripatetic distinction wherever it has diagnostic value, and does not insist on the Stoic systematisation when it would obscure a particular analysis.

The Theophrastean logic: an offence born of θυμός involves grief (λύπη) and is reactive (caused by a prior injustice); an offence born of TERMἐπιθυμία involves pleasure (ἡδονή) and is initiated by the agent himself. The first is therefore closer to the ἀκούσιον (involuntary), the second to the ἑκούσιον (voluntary); the voluntary offence is graver. This Aristotelian distinction ἑκούσιον / ἀκούσιον, taken at a Theophrastean angle, gives Marcus an ethical scale. See Med. 11.18, where Marcus unfolds a therapy specifically directed against θυμός — in nine points, a separately focused text on the management of anger.