Along with Leucippus, the founder of classical atomism. He was known as very widely travelled, and was called the laughing philosopher. There is a legend, related by Tertullian although denied by Plutarch, that in order to retain his cheerfulness (euthumia, the ultimate end recommended by his ethical system) he blinded himself, so as to put an end to his desire for women (see Ulysses). The atomism proposed by Democritus and Leucippus was a response to the Eleatic arguments against motion. The Eleatics argued that what is real is both single and motionless, since motion is impossible without empty space (see also Zeno’s paradoxes) and plurality is impossible without empty space to separate the different unities. By allowing empty space, the atomists could avoid the Eleatic conclusion, but the individual atoms retain the characteristics that Parmenides attributed to the whole of unchanging reality. They are indivisible, homogeneous, solid, and unchanging, but they may differ from each other in shape and size. They are infinite in number, exist in empty space (the void), and are in eternal motion. When enough atoms exist in a region of space they form a vortex, with a mass of heavier atoms at the centre attracting others; the speed of the motion ignites such masses and causes the celestial bodies. The arrangements and conglomerations of atoms produce the world we experience; this world is, however, only one of the infinite number of worlds that different arrangements of atoms produce. The soul is made of particularly fine atoms, but is a composite and hence as perishable as the body. Perception is the result of eidola or thin films of atoms being shed from the surfaces of objects and interacting with the atoms of the soul.

The magnificent vision of the universe that Democritus conjures up, with its mechanism and its total absence of purpose and design, was too much for Plato and Aristotle, and only wholeheartedly embraced by Epicurus, until it was rediscovered in the 17th century and formed the basis of modern science.