The Milesian School is an early Pre-Socratic school of philosophy founded in the 6th Century B.C. in the Ionian town of Miletus (a Greek colony on the Aegean coast of Anatolia in modern Turkey). The major philosophers included under this label are Thales, Anaximander and Anaximenes, who held quite distinct views on most subjects, so that the grouping is more one of geographical convenience than one of shared opinions (although it is thought likely that Thales taught Anaximander, who in turn taught Anaximenes).
Arguably, it forms part of the Ionian School, which additionally includes Heraclitus, Anaxagoras, Diogenes Apolloniates (c. 460 B.C.), Archelaus (5th Century B.C.) and Hippo (5th Century B.C.), although this larger group has even fewer points of affinity. The Milesians were also more focused on nature than on reason and thought like the later Ionians.
The Milesians introduced new opinions (contrary to the then prevailing views) on how the world was organized, in which natural phenomena were explained solely by the will of anthropomorphized gods (with human characteristics). They are sometimes described as philosophers of nature, and they presented a view of nature in terms of methodologically observable entities, and therefore represented one of the first attempts to make philosophy truly scientific.
In Metaphysics, they defined all things by their quintessential substance ("archê"), of which the Universe was formed and which was the source of all life (Materialistic Monism). However, they differed widely in how they conceived of this substance: Thales thought it was water; Anaximander called it "apeiron" (something infinite and indeterminate); Anaximenes settled on air. In general, they believed in hylozoism, the idea that all life is inseparable from matter, and that there is no distinction between the animate and the inanimate, between spirit and matter.
In cosmology, they also differed in the way they conceived of the universe: Thales believed that the Earth was floating in water; Anaximander placed the Earth at the center of a universe composed of hollow, concentric wheels filled with fire, and pierced by holes at various intervals (which appear as the sun, the moon and the stars); Anaximenes saw the sun and the moon as flat disks traveling around a heavenly canopy, on which the stars were fixed.
