 The early Greek atomists try to account for the formation of the natural world by means of their simple ontology of atoms and void alone. Leucippus held that there are an infinite number of atoms moving for all time in an infinite void, and that these can form into cosmic systems or kosmoi by means of a whirling motion which randomly establishes itself in a large enough cluster of atoms. It is controversial whether atoms are thought to have weight as an intrinsic property, causing them all to fall in some given direction, or whether weight is simply a tendency for atoms (which otherwise move in any and every direction, except when struck) to move towards the centre of a system, created by the whirling of the cosmic vortices. When a vortex is formed, it creates a membrane of atoms at its outer edge, and the outer band of atoms catches fire, forming a sun and stars. These kosmoi are impermanent, and are not accounted for by purpose or design. The earth is described as a flat cylindrical drum at the center of our cosmos.