On the Meaning of στερεόν in Democritus' Theory of Vision

Among the technical terms employed to characterize atoms as fundamental elements of reality, ancient sources also preserve the term στερεόν. Both Democritus and Epicurus associate this term with the materiality of par- ticles, and scholars generally render it as ‘solid’ or similar expressions such as ‘compact’ or ‘hard’. While στερεόν is predominantly employed to denote the concrete compactness of atoms and to imply a reference to their indivisibility, Theophrastus’ De sensu 50 (68 A 135 DK), which preserves the most comprehen- sive account of Democritus’ theory of vision, also applies the term to the con- troversial notion of air-imprints, namely images formed in the air that enable our visual perception. This paper examines Theophrastus’ testimony and pro- poses a geometric interpretation of στερεόν as ‘three-dimensional’.