The apeiron is a cosmological theory created by Anaximander in the 6th century BC. Anaximander's work is mostly lost. From the few extant fragments, we learn that he believed the beginning or first principle (arche) is an endless, unlimited mass (apeiron), subject to neither old age nor decay, which perpetually yields fresh materials from which everything which we can perceive is derived. The apeiron was never defined precisely, and it has generally (e.g. by Aristotle and Augustine) been understood as a sort of primal chaos. It embraced the opposites of hot and cold, wet and dry, and directed the movement of things, by which there grew up all of the host of shapes and differences which are found in the world.
Out of the vague and limitless body there sprung a central mass -- this earth of ours, cylindrical in shape, poised equidistant from surrounding orbs of fire, which had originally clung to it like the bark round a tree, until their continuity was severed, and they parted into several wheel-shaped and fire-filled bubbles of air. Man himself and the animals had come into being by like transmutations. Mankind was supposed by Anaximander to have sprung from some other species of animals, probably aquatic. But as the measureless and endless had been the prime cause of the motion into separate existences and individual forms, so also, according to the just award of destiny, these forms would at an appointed season suffer the vengeance due to their earlier act of separation, and return into the vague immensity whence they had issued. Thus the world, and all definite existences contained in it, would lose their independence and disappear in the "indeterminate." The blazing orbs, which have drawn off from the cold earth and water, are the temporary gods of the world, clustering round the earth, which, to the ancient thinker, is the central figure.
Other pre-Socratic philosophers had different theories of the apeiron. For Pythagoras, the universe had begun as an apeiron, but at some point it inhaled the void from outside, filling the cosmos with vacuous bubbles that split the world into many different parts. For Anaxagoras, the initial apeiron had begun to rotate rapidly under the control of a godlike Nous (Mind), and the great speed of the rotation caused the universe to break up into many fragments. However, since all individual things had originated from the same apeiron, all things must contain parts of all other things-- for instance, a tree must also contain tiny pieces of sharks, moons, and grains of sand. This alone explains how one object can be transformed into another, since each thing already contains all other things in germ.