Democritus (mid 5th–4th century BC)

A co-founder with Leucippus of the theory of atomism, The Greek Philosopher Democritus developed it into a universal system, embracing physics, cosmology, epistemology, psychology and theology. He is also reported to have written on a wide range of topics, including mathematics, ethics, literary criticism and theory of language. His works are lost, except for a substantial number of quotations, mostly on ethics, whose authenticity is disputed. Our knowledge of his principal doctrines depends primarily on Aristotle’s critical discussions, and secondarily on reports by historians of philosophy whose work derives from that of Aristotle and his school. The atomists attempted to reconcile the observable data of plurality, motion and change with Parmenides’ denial of the possibility of coming to be or ceasing to be. They postulated an infinite number of unchangeable primary substances, characterized by a minimum range of explanatory properties (shape, size, spatial ordering and orientation within a given arrangement). All observable bodies are aggregates of these basic substances, and what appears as generation and corruption is in fact the formation and dissolution of these aggregates. The basic substances are physically indivisible (whence the term atomon, literally ‘uncuttable’) not merely in fact but in principle; (1) because (as Democritus argued) if it were theoretically possible to divide a material thing ad infinitum, the division would reduce the thing to nothing; and (2) because physical division presupposes that the thing divided contains gaps. Atoms are in eternal motion in empty space, the motion caused by an infinite series of prior atomic ‘collisions’. (There is reason to believe, however, although the point is disputed, that atoms cannot collide, since they must always be separated by void, however small; hence impact is only apparent, and all action is at a distance.) The void is necessary for motion, but is characterized as ‘what-is-not’, thus violating the Eleatic principle that what-is-not cannot be. Democritus seems to have been the first thinker to recognize the observer-dependence of the secondary qualities. He argued from the distinction between appearance and reality to the unreliability of the senses, but it is disputed whether he embraced scepticism, or maintained that theory could make good the deficiency of the senses. He maintained a materialistic account of the mind, explaining thought and perception by the physical impact of images emitted by external objects. This theory gave rise to a naturalistic theology; he held that the gods are a special kind of images, endowed with life and intelligence, intervening in human affairs. The ethical fragments (if genuine) show that he maintained a conservative social philosophy on the basis of a form of enlightened hedonism.