Many of the early natural philosophers from Miletus attempted to provide theories about the origins of the universe. They sought natural descriptions of the physical world. They wished to find the stuff which makes up all other things.
On the one hand, Thales conceived of the material world as water existing in various densities. Curd records Aristotle's description of Thales' philosophy, writing, "For there must be one or more natures from the rest come to be, while it is preserved. However, they do not all agree about how many or what kinds of such principles there are, but Thales...stated it to be water" (Curd, 15). For Thales, the material world is essentially water. This one element, he thinks, is the source of the other elements.
On the other hand, Anaximander argues that the first principle is indefinite, indestructible, and infinite. He held that "the arhke is neither water nor any of the other things called elements, but some nature which is apeiron, out of which come to be all the heavens and the worlds in them" (Curd, 17). This indefinite cause brings dualities such as hot and cold into existence. These then result in the material world, including fire and the stellar world.
Unlike Anaximander, Anaximenes thought that the underlying stuff of the cosmos was an element. "Anaximenes...like Anaximander, declares that the underlying nature is one and unlimited [apeiron] but not indeterminate, as Anaximander held, but definite, saying that it is air" (Curd, 19). Thus he synthesizes the elemental explanation of Thales and the apeiron of Anaximander to come to his own theory. He finds that air is the unlimited substance of the cosmos.