Thales of Miletus, who is recognized as the first philosopher because of his reliance upon logos, or reason, to understand the world, articulated a philosophical system the framework of which came to be known as "The Milesian Picture." In this system, the Universe has order because there is one source, one unity, which underlies and unites the multiplicity of individual, transitory things in the world. These transitory things pass into and out of existence, while the underlying source of existence itself, known as the arche, Being itself, endures, infinitely, eternally, and unchanged. According to Thales, the arche of all transitory beings is water. It is believed that Thales reached this conclusion empirically, that is, through simple observation of the world around him. Such observable facts as all living beings' need for water to continue living and the appearance that the island on which he lived floated on the sea probably lead Thales to this conclusion.
Thales' successor Anaximander articulated his philosophical system within the framework of the Milesian Picture as well. However, Anaximander disagreed with Thales that water was the arche. According to Anaximander, water cannot be the unalterable source behind all beings, because it itself is a definite substance, a being. No finite, changeable substance can be the unalterable, eternal Being. Instead, Anaximander argued that apeiron, or, the limitless, an infinite substance which, as such, is beyond the perception and descriptive abilities of finite human minds, must be the source of all beings.
Anaximander's position is far stronger than Thales, even though it leads one to a rather problematic conclusion. If Being, the source of all beings, must be infinite and unchangeable (requirements which Thales himself put forth), then it is impossible for water to be the arche. Water is alterable; one may freeze it or cause it to evaporate, or even, as we now know, destroy it completely by breaking it down into its component atoms of hydrogen and oxygen. Water is finite. Being itself, the arche, could not truly be the infinite source of all finite beings if it were alterable and/or destroyable. Anaximander's apeiron may be understood as a convenient label for a substance which humans, as finite beings which pass to and from existence, cannot understand. However, it indicates that this substance is infinite and, as such, unalterable. Thus it fits the conditions for the ultimate source behind all finite beings. This does, admittedly, lead one back to the rather problematic conclusion that a complete understanding of the universe cannot, at bottom, be obtained solely through rational thought, though one can still logically reason one's way to this conclusion, rather than just making up stories to explain the world.