Anaximander

Anaximander was younger contemporary and a pupil of Thales. He agreed with Thales that there is some single basic stuff out of which everything comes. Unlike Thales, however, Anaximander said that this basic stuff is neither water nor any other specific of determinate element, arguing that water and all other definite things are only specific variations of something which is more primary.  It may very well be that water or moisture is found in various forms everywhere, but water is only one specific thing among many other elements, and all these specific things require that there be some more elementary stuff to account for their origin. The primary substance out of which all these specific things come, said Anaximander, is an "indefinite or boundless realm." Thus, Anaximander differentiates specific and determinate things from their origin by calling the primary substance the indeterminate boundless. But actual things are specific, their source is indeterminate, and whereas things are finite, the original stuff is infinite or boundless. 

Besides offering a new idea about the original substance of things, Anaximander advanced the enterprise of philosophy by attempting some explanation for his new idea. Thales had not dealt with the problem of explaining how the primary stuff became the many different things we see in the world, but Anximander addressed himself precisely to this question. Although his explanation may seem strange, it represents an advance in knowledge in the sense that it is an attempt to deal with known facts from which hypotheses can be formulated instead of explaining natural phenomena in mythical and nondebatable terms. What Anximander has to say about the origin of things he speaks of this unoriginated and indestructible primary substance as also having eternal motion. As a consequence of this motion, the various specific elements come into being as a "separating off" from the original substance, and thus, "there was an eternal motion in which the heavens came to be." But first warm and cold were separated off, and from these two came moist; then from these came earth and air. Anaximander then tried to account for the heavenly bodies and air currents around the earth in what appears to be a mechanical explanation of the orderly movement of the stars. He thought the earth was cylindrical in shape in contrast to Thales, who thought it was flat as a disk and floated on the water. 

Returning again to the vast cosmic scene, Anaximander thought that there were many worlds and many systems of universes existing all at the same time, all of them perishable, there being the constant alternation between their creation and destruction. This cyclical process was for him a rigorous necessity as the conflict of opposite forces in nature caused what he called poetically an "injustice" requiring their ultimate destruction. In the only sentence from his writings that has survived, Anaximander gathers up his chief thought by saying that "From what source things arise, to that they return of necessity when they are destroyed; for they suffer punishment and make reparation to one another for their injustice according to the order of time."

Fragment

The Unlimited is the first-principle of things that are. It is that from which the coming-to-be (of things and qualities) takes place, and it is that into which they return when they perish, by moral necessity giving satisfaction to one another and making reparation for their injustice, according to the order of time.