On Forms (Peri Ideōn)

Major atomist text — atomic shapes (ideai) as the source of perceptible qualities.

"What appears as differences of taste, colour, sound — these are differences of the shapes and arrangements of the underlying atoms." (Democritus, On Forms, reconstructed via Theophrastus)

Naturalist-philosophical reduction of perceived qualities to underlying atomic-geometrical structure.

"What the senses report is the consequence, in our soul-atoms, of the interaction of the eidola with our perceptual organs; the underlying reality is purely geometrical-atomic." (Democritus, On Forms, reconstructed)

Major pre-Socratic physical-philosophical text.

"On Forms develops the atomic-cosmological framework into systematic account of perceptible qualities — among the most ambitious pre-Socratic philosophical achievements." (Standard scholarly account)

Realist about atomic geometry; conventionalist about perceived qualities.

"Two forms of knowledge: the legitimate apprehends atoms and void; the bastard knows only the conventional appearances." (Democritus, fragment B11)

Major source for Epicurean atomic theory of qualities — Lucretius preserves the framework.

"The Epicurean-Lucretian account of perceptible qualities arising from atomic-geometrical structures is in deep continuity with Democritus's On Forms." (Standard scholarly account)

Foundational text for the primary-secondary-quality distinction central to early-modern philosophy (Galileo, Descartes, Locke) and contemporary analytic metaphysics.

"The Democritean primary-secondary-quality distinction — atomic-geometrical reality vs. conventional perceived qualities — is the ancient anticipation of the early-modern and contemporary distinction." (Standard scholarly account)