Greek philosopher credited by Aristotle and by Theophrastus with having originated the theory of atomism. It has been difficult to distinguish his contribution from that of his most famous pupil, Democritus. Only fragments of Leucippus' writings remain, but two works believed to have been written by him are The Great World System and On the Mind. His theory stated that matter is homogeneous but consists of an infinity of small indivisible particles. These atoms are constantly in motion, and through their collisions and regroupings form various compounds. A cosmos is formed by the collision of atoms that gather together into a “whirl,” and the drum-shaped Earth is located in the centre of man's cosmos.