Democritus of Abdera was a natural philosopher, founder — together with his teacher Leucippus — of atomism. He came from a wealthy family and, by tradition, spent his inheritance on travels (Egypt, Babylon, Persia, perhaps India) in the pursuit of knowledge. An extraordinarily prolific author (Thrasyllus grouped his works into tetralogies, as he did Plato's), but almost all is lost — only fragments survive, chiefly ethical. He was nicknamed the "laughing philosopher" — in the later tradition he was set against the "weeping" Heraclitus.

The principal biographical source is Diogenes Laertius IX 34–49. There are competing versions of his death, and Rogovin's note to 03-03 names both: by Diogenes Laertius, Democritus died of extreme old age (living, on some accounts, past a hundred); by Lucretius (De rer. nat. III 1039–1041), he ended his own life on sensing the approach of senile decay. Marcus gives a folk, deflating version: he was "eaten away by lice" (φθεῖρες).

Atomism. Reality consists of an infinite multitude of indivisible bodies — atoms (ἄτομα, "uncuttable"), differing in shape, arrangement, and position — moving in the void (κενόν). The coming-to-be and passing-away of things is merely the combination and separation of atoms; qualities (colour, taste, heat) exist "by convention" (νόμῳ), while "in truth" (ἐτεῇ) there are only atoms and void. Worlds arise from atomic vortices without design or purpose — pure mechanical necessity, without providence.

Ethics. The goal of life is εὐθυμία (good cheer, "well-being of soul"), attained through measure and equanimity; a forerunner of Epicurean ἀταραξία and Stoic apatheia.

Why he matters for Marcus. Democritean (and the Epicurean that follows it) atomism is the chief rival cosmology to Stoic providence. Hence the recurring fork in Marcus: ἤτοι πρόνοια ἢ ἄτομοι — "either providence or atoms." Again and again Marcus sets the reader before this choice: the ordered, rational, providential cosmos of the Stoa — or the blind concourse of Democritus' atoms. Notably, Marcus does not always "close" the dilemma by force: at times he shows that even on the atomist hypothesis the Stoic ethical conclusion (accept what happens, hold fast to your own reason) still stands. Thus for Marcus Democritus is not simply mistaken but a standing mental opponent, a test of the soundness of the Stoic position.

• 03-03 — death "by lice" in the catalogue of famous deaths (beside Socrates, whom "other lice" — human ones — killed).
• 04-24 — Marcus quotes and refines Democritus's εὐθυμία-maxim ("Ὀλίγα πρῆσσε…", DK 68 B3): not "do few things" but "do what is necessary and socially due."
• 07-31 — a recasting of the "νόμῳ… ἐτεῇ" thesis (DK 68 B9/B117): "all by convention, in reality only the elements." Democritus goes unnamed (ἐκεῖνος, "that man"; Rogovin — "the poet"), but the fragment is recognizable; Marcus turns the atomist's reduction into a prop of indifference.
• The fork "providence or atoms" (πρόνοια ἢ ἄτομοι) — Med. 4.3; 06-10; 07-32; 8.17; 9.28; 9.39; 10.6; 11.18; 12.14. Democritus is usually not named here, but it is his system that is the "atomist" horn of the dilemma.