Kakia is the Stoic technical term for vice, the polar opposite of TERMvirtue. Where ἀρετή is διάθεσις τοῦ TERMἡγεμονικοῦ κατὰ λόγον — the disposition of the ruling part in accordance with reason — κακία is διάθεσις τοῦ ἡγεμονικοῦ παρὰ λόγον — its disposition contrary to reason. It is a state of the soul, not a single act (a single act is a ἁμάρτημα).

The four cardinal κακίαι are parallel to the four cardinal virtues:

As with virtue, kakia in the Stoics obeys the thesis of unity: a person vicious in one respect is vicious in all, because the four κακίαι are aspects of a single condition — disagreement with the Logos. All κακίαι are ψευδεῖς ὑπολήψεις — false evaluative judgments about good and evil.

SVF III 91–105 (Chrysippus on κακία as a state); III 256, 262 (passions as κρίσεις, vice as a settled false disposition); DL VII 93, 100–101; Stob. Ecl. II 58–60, 96 W; Cic. De fin. III 39; Tusc. IV 14, 28–32; LS 61. Kakia is not very frequent in Marcus (he prefers TERMkakon and TERMpathos), but appears in important places: Med. 2.1 (vice as ignorance of good and evil); 4.49; 9.42; 12.27.

The radical Stoic thesis: between ἀρετή and κακία there is no middle ground. All human beings divide into sages (who possess all four virtues in full) and non-sages (who possess all four vices). There is no "in-between" (μέσος): the Stoics reject Aristotle's μεσότης (the doctrine of virtue as a mean). This yields a paradoxical consequence: almost everyone is vicious in the Stoic sense, because a sage appears more rarely than the phoenix (Sen. Ep. 42; SVF III 657).

The doctrine is softened by προκοπή — the gradual "progress" from folly toward wisdom. Most Stoics regarded themselves as προκόπτοντες — those making progress, not yet sages but moving in that direction. See Epict. Disc. 1.4 in full on προκοπή.

In Marcus, kakia occurs in the context of ignorance of TERMgood and TERMevil (02-01): vice is the consequence of incorrect evaluative knowledge, and therefore the right response to a vicious person is not anger but pity for his blindness (see also the image of moral blindness in 02-13).