Democritus

Democritus was born at Abdera on the Thracian coast. He reportedly inherited a fortune, travelled through Egypt, Persia and perhaps India, spent it all, and returned home famous for learning. He lived past ninety and was cheerful by temperament; later ages called him the laughing philosopher, paired with Heraclitus the weeping one. He wrote over sixty works on physics, mathematics, ethics, music and farming, all lost.

Atomism was begun by his teacher Leucippus and completed by him. It is the most thorough answer to Parmenides: accept that Being neither comes to be nor perishes, but let there be countless beings instead of one. Atoms are indivisible particles differing in shape, size and arrangement, moving eternally in the void, colliding and hooking together to form all things. The void is 'what is not', yet it is just as real. The soul too is made of the finest spherical atoms, and death is their scattering.

He counted colour, taste, hot and cold as conventions; only atoms and void are real. This is the earliest distinction between primary and secondary qualities. His ethics recommended 'cheerfulness', a stable state produced by moderation and calm of mind. Plato is said to have wanted to burn all his books; Epicurus took over his physics.