Milesian school is an ancient Greek philosophical school, founded by Thales in VI BC in Miletus, one of the cities of Ionia. Its most famous representatives are Thales, Anaximander and Anaximenes. They introduced new opinions contrary to the prevailing belief of how the world was organized, in which natural phenomena were explained solely by the will of anthropomorphized gods. The Milesians conceived of nature in terms of methodologically observable entities, and as such was one of the first truly scientific philosophies.

Sometimes it is included to the Ionian philosophy, the concept of which is introduced by Diogenes Laertius. To the Ionian philosophers belonged the disciples of Thales and the disciples of his disciples: Anaximenes, Anaximander, Anaxagoras, Archelaus. Ionian philosophy refers to the early (pre-Socratic) period of the formation of Greek philosophy and was characterized by an interest in natural-science problems (natural philosophy).

Natural philosophy became the first philosophical doctrine of ancient Greece, which began to examine moral issues. It came out of Greek mythology, but unlike it, the question was not about the one who gave birth to all that exists, but from what it came out, with almost no consideration of the moral side of human existence.

Milesian school was primarily focused on natural science and did not pose the theoretical problems of being and cognition (therefore it is more correct to talk about its "naturalism"); the history of European scientific cosmogony and cosmology, physics, geography (and cartography), meteorology, astronomy, biology and (possibly) mathematics begins with this school. All this constituted a single science about nature, or natural history, which describes and explains the cosmos in its evolutionary dynamics: from the origin of the stars and land from the primacy to the appearance of living beings (in Anaximander’s teachings). The law is eternal, infinite in space, the movement is inherent in it from, the world is born from it spontaneously (perhaps through the cosmogonic vortex).

The "gods" of folk mythology are identified with the elements and luminaries (Anaximenes) or "countless worlds" (Anaximander), which originated from a single propriety, which itself is conceived as a higher and absolute "deity". The pantheism of the Milesian school was more naturalistic (in contrast to Heraclitus). The Milesian school for the first time abolished the mythological picture of the world, based on the opposition of the heavenly (divine) to the earthly (human), and introduced the universality of physical laws.

Milesian school regarded the world as a living whole; philosophers did not make a fundamental difference between the living and the dead, the mental and the physical, recognized for inanimate objects only a lesser degree of animation (life). The animation itself (soul) was regarded as a "subtle" and mobile form of primordiality.