Democritus (c. 460-370 BC) was an ancient Greek scientist and materialist philosopher, the chief representative of ancient atomism. According to Democritus, matter consists of a countless number of minute indivisible particles — atoms — which, by joining and separating, form the whole boundless variety of things in nature. Atoms are eternal and unchanging and differ in shape and in size. He held that there exists in the Universe a countless number of worlds, which arise, develop and perish.