Democritus
Notes
Democritus (c. 460–c. 370 BC), Greek philosopher from Abdera and, with his teacher Leucippus, the originator of atomism: the world consists of indivisible atoms moving in the void, and everything else, including the soul and the senses, is their arrangement. "By convention sweet, by convention bitter; in truth, atoms and void." This is the first statement of materialism in the Western tradition, and Epicurus and Lucretius took it from him. Known as "the laughing philosopher", which seems the right response to having been correct two and a half thousand years early.
Wikipedia: https://en.wikipedia.org/wiki/Democritus