Democritus c. 460 BC – c. 370 BC

Democritus of Abdera was a Pre-Socratic Greek philosopher of the fifth and early fourth centuries BC, regarded with his teacher Leucippus as a co-founder of the atomist tradition in Greek thought. He held that the entire universe consists of indivisible material particles, atoms, moving in an infinite void, and that the perceptible qualities of things arise from the arrangement and motion of those atoms. He wrote prolifically across ethics, physics, mathematics, music, and biology, but only fragments of his works survive in quotations from later authors. His ethical fragments, dealing with cheerfulness as the goal of life and the relation of pleasure to virtue, anticipate Hellenistic Epicureanism. Tradition called him the Laughing Philosopher for his cheerful temperament.

Democritus (c. 460–370 BC) was the most systematic developer of pre-Socratic atomism and one of the most influential pre-Socratic philosophers. Born in Abdera in Thrace, he traveled extensively for philosophical instruction — Egypt, Persia, possibly India, on the traditional accounts — and produced an enormous body of work, of which only fragments and reports survive.

Democritus, working from the foundations laid by his teacher Leucippus, developed the doctrine that all of reality is composed of indivisible atoms moving through void. The variety of perceptible qualities — color, taste, warmth — is reducible to the geometrical and kinetic properties of atoms; sweetness and color exist only by convention, while atoms and void exist by nature. Democritus's atomism is the most thoroughgoing materialist physics produced before the early modern period.

Democritus's ethical fragments — he was nicknamed the laughing philosopher in antiquity for his good humor and equanimity — develop an early eudaimonist position grounded in the cultivation of cheerfulness (euthumia) and moderation. Epicurus took up Democritean atomism a century later and developed it into a comprehensive philosophical school. The early modern recovery of atomism through Lucretius's transmission of Epicureanism returned Democritus to philosophical prominence; Galileo, Boyle, and Newton all worked within an explicitly Democritean framework against the Aristotelian inheritance.