Socrates was an Athenian, son of the stonemason Sophroniscus and the midwife Phaenarete; in youth he seems to have practised his father's trade. He served as a hoplite (Potidaea, Delium, Amphipolis), distinguished for courage. He was married to Xanthippe. He wrote nothing himself: all we know comes through his pupils — above all Plato (the dialogues) and Xenophon (Memorabilia, Apology), and through the caricature of Aristophanes (Clouds).

His life was conversation: in the Athenian agora he subjected his fellow-citizens to the elenchus (the testing cross-examination), exposing the emptiness of their supposed "knowledge." The Delphic oracle called him the wisest of men — which Socrates construed thus: he is wiser only in knowing that he does not know. He was guided by a δαιμόνιον — an inner divine "voice" that held him back from wrong acts (a link to δαίμων).

In 399 BCE he was charged by Anytus, Meletus, and Lycon with impiety and the corruption of the youth, condemned, and executed by hemlock. He refused to escape (Crito) and met death with composure (Phaedo). In 03-03 Marcus calls his destroyers "other lice" — human parasites, as against the literal lice that consumed Democritus.

Socrates effected the turn of philosophy from nature to ethics: the central question is no longer "of what is the world made" but "how ought one to live." His key positions:

• Virtue is knowledge; no one does wrong willingly (moral intellectualism): a bad act follows from ignorance of the true good.
• Care of the soul (ἐπιμέλεια ψυχῆς) matters more than care for the body, property, reputation.
• "The unexamined life is not worth living" (Apology 38a).
• Moral self-sufficiency: to the good man "no evil can come, either in life or after death" (Apology 41d) — the direct source of the Stoic doctrine "no harm can be done to the virtuous".

For the Stoics Socrates is the archetype of the sage (σοφός). The line of descent: Socrates → the Cynic Antisthenes (his pupil) → Diogenes → Crates → Zeno of Citium. The Stoics saw in Socrates a perfect model: equanimity in the face of death, indifference to externals, fidelity to reason and to the inner voice, virtue as the only good. Socrates' calm end is the benchmark of the Stoic stance toward death.

Strikingly, the disjunction on death in 03-03 (either another life or insensibility — and neither is to be feared) reproduces Socrates' own argument in the Apology 40c–41c (death is either a dreamless sleep or a migration of the soul). Here Marcus thinks in literally Socratic terms.

• 03-03 — death "by other lice" (his accusers) in the catalogue of famous deaths; and the disjunctive consolation that goes back to Socrates' Apology.
• 03-06 — an explicit attribution: the daimon has detached itself from the persuasions of the senses, "as Socrates said" (ὡς ὁ Σωκράτης ἔλεγεν) — a reference to the Platonic Phaedo (65a–67b: philosophy as the soul's withdrawal from the body and the senses), folded into the Stoic discipline of assent.
• Med. 1.16 — the virtues of Antoninus Pius are described after a Socratic model.
• 06-47 — Socrates in the roll-call of the dead (beside Heraclitus and others).
• 07-66 — an extended reflection: Socrates' outward feats (the noble death, the disputes, the night in the frost, the refusal to arrest Leon, the swaggering gait) are insufficient; the measure is what ψυχή he had (just toward men, pious toward the gods, no slave to another's ignorance).
• 07-19 — "how many Socrateses has eternity already swallowed": the name in the plural, in the roll-call of the greatest beside Chrysippus and Epictetus; 7.66 — the true greatness of Socrates (the anecdote from Xenophon on his endurance and self-command).
• 08-03 — Socrates, Diogenes, Heraclitus set against Alexander, Caesar, Pompey: the sages against the conquerors.
• Med. 11.23, 11.25, 11.28 — Socratic anecdotes and sayings.