In Stoic technical usage, πάθος is not merely "emotion" but specifically an irrational emotion: "a movement of the soul that is contrary to nature and irrational" (Chrysippus, Stob. Ecl. II 88). A πάθος arises from the assent (συγκατάθεσις) given by the TERMruling part to a false evaluative TERMimpression — for example, to an impression "this [external good] is a real good" or "this [non-vice] is a real evil." The Stoics identify four cardinal πάθη along two axes (good/evil × present/future): ἐπιθυμία (desire — apparent good in the future), φόβος (fear — apparent evil in the future), ἡδονή (pleasure — apparent good in the present), λύπη (distress — apparent evil in the present). The ideal is TERMἀπάθεια — not insensibility, but freedom from false assents.

SVF III 377–490 (the large corpus on the πάθη — the principal Stoic psychological theory); classical formulations in DL VII 110–115; Stob. Ecl. II 88–93 W; Cic. Tusc. III–IV; LS 65. In Marcus: Med. 2.5; 2.10; 05-26 (add to the sensation no judgment of "good/bad"); 6.16; 8.29; 11.18.

In 02-05 πάθος appears in adjectival form as ἐμπαθής: «freed from … ἐμπαθοῦς ἀποστροφῆς ἀπὸ τοῦ αἱροῦντος λόγου» — "from passion-charmed turning-away from the governing reason." Passion is here defined by its structural effect: it tears the agent away from its own TERMreason. This is the typical Stoic formulation: πάθος is not "something added to reason" but reason's inner rift, the splitting of the self from itself. The therapy is not to suppress the emotion but to track the act of assent from which it is born (see the exercises prosochē and physical definition): a thing correctly described does not provoke passion.