Apatheia literally means "without-passion" (from ἀ- + TERMπάθος). It is the Stoic technical term for the state of the sage: a soul free from irrational passions. The aim of Stoic ethics is not the suppression of feeling but the elimination of false evaluative assents from which TERMπάθη arise.

The key distinction: apatheia ≠ "insensibility" (ἀναισθησία). The sage is not a wooden statue. He:

• experiences εὐπάθειαι — rational affective states (TERMχαρά, βούλησις, εὐλάβεια) in response to the genuine good;
• experiences προπάθειαι — "pre-passions," involuntary first reactions of the body (a start at a loud noise, pallor at danger) that are not passions and do not depend on assent;
• does not assent to false evaluative impressions, and therefore has no ἐπιθυμία, ἡδονή, φόβος, λύπη.

A subtlety: apatheia does not mean "I feel no pain." A sage under torture does feel physical pain (πόνος) as a disturbance in the body. But he does not append to that sensation the evaluative judgment "this is evil" — and so there is no λύπη (the passion of distress) in his soul. The pain remains sensation; it does not become a passion.

Polemic with Aristotle: the Peripatetics (Aristotle, EN II 6) teach μετριοπάθεια — moderation of the passions, keeping them within their proper bounds. The Stoics reject this: passions are errors of reason (false κρίσεις), and therefore are to be eliminated entirely, not moderated. A "moderate" illness is still an illness.

SVF III 200, 444–449 (Chrysippus on apatheia); DL VII 117 (the definition); Stob. Ecl. II 95–96, 103 W; Cic. Tusc. III 12–13, IV 38–58 (the Peripatetic polemic against Stoic apatheia); Sen. Ep. 9 (a defence of the sage against the charge of "insensibility"); LS 65, 66. The distinction προπάθεια vs πάθος: Sen. De ira II 3–4; Aulus Gellius NA XIX 1 (the famous episode of the storm and the Stoic).

The term apatheia is not used in Marcus, but the concept is central: the whole of his meditative work runs against the TERMpassions and toward the installation of their eupatheia-correlates. See especially Med. 11.18 — the nine-point "therapy of anger."

The word apatheia had a complicated history and was already prone to misunderstanding in antiquity. Cicero reports the criticism that the Stoics preach insensibility (insensibilitas, Tusc. III 12) — and proceeds to refute it himself by explaining the distinction between passions and ordinary emotions.

In early Christianity apatheia was reinterpreted: in Clement of Alexandria, Origen, and later in the Egyptian monks (Evagrius Ponticus, Praktikos), apatheia becomes the highest spiritual goal — a state in which the passions are weakened or removed, and the soul is open to contemplation (θεωρία). This is no longer quite the Stoic apatheia — for Christians it is bound up with grace and ascetic effort, for the Stoics with right dialectic and exercises. But the terminological continuity is plain.

In modern English, "apathy" — and Russian апатия — means the opposite: indifference, listlessness, depressive detachment. This is the complete antonym of Stoic apatheia. The ancient sage is not apathetic — he is calmly energetic, actively engaged, but not governed by false assents. The Stoic apatheia is therefore best rendered in English not as "apathy" but as "freedom from passion" or "imperturbability".

See also the self-sufficiency of virtue: apatheia is its consequence. The sage, possessing the only genuine good (virtue), is disturbed by nothing external, and so has no passions.