Thales, Anaximander, and Anaximenes (sixth century B.C.E.)—all from Miletus—are often identified as the earliest Greek philosophers and cosmologists. This identification is partly due to Aristotle (384–322 B.C.E.), who presented them as "fore-runners" for his own physical theories. Aristotle is also the main source for their work, so his accounts must be treated with care as they are frequently colored by his theories. More generally, because so little is known about them, the Milesians have proved a malleable material to later thinkers in search of a Greek origin for their discipline.
The Milesians dealt both with natural phenomena, such as earthquakes and lightning, and with the structure of the cosmos, for instance how the earth is supported. Their explanations refer to the physical properties of things, but often rely on reasoning rather than observation. Thales supposedly argued that the earth is supported by water, while Anaximander stated that the earth rests in the middle of the cosmos, because it has no more reason to go in one direction than another. Aristotle identified a fundamental physical entity in each of the thinkers' theories: water, the boundless, and air, respectively. It is unlikely, however, that all the Milesians used these entities as material building blocks of the cosmos in the way that Aristotle envisaged.
The theories of the Milesians are often contrasted with mythological accounts found in Hesiod and Homer. However, while the Milesians do not refer to actions of the gods in their cosmologies, their theories owe much to earlier Greek and near-Eastern myths and their explanations are speculative.
The Milesians were followed by an array of thinkers criticizing and developing their thoughts. Among these were the Atomists, who argued for an infinte cosmos consisting of atoms and the void, and governed only by material interaction.