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Milesian school

Thales of Miletus, regarded by Aristotle as the first philosopher, held that all things arise from a single material substance, water. It is not because he gave a cosmogony that John Burnet calls him the "first man of science", but because he gave a naturalistic explanation of the cosmos and supported it with reasons. According to tradition, Thales was able to predict an eclipse and taught the Egyptians how to measure the height of the pyramids.

Thales inspired the Milesian school of philosophy and was followed by Anaximander, who argued that the substratum or arche could not be water or any of the classical elements but was instead something "unlimited" or "indefinite" (in Greek, the apeiron). He began from the observation that the world seems to consist of opposites (e.g., hot and cold), yet a thing can become its opposite (e.g., a hot thing cold). Therefore, they cannot truly be opposites but rather must both be manifestations of some underlying unity that is neither. This underlying unity (substratum, arche) could not be any of the classical elements, since they were one extreme or another. For example, water is wet, the opposite of dry, while fire is dry, the opposite of wet. This initial state is ageless and imperishable, and everything returns to it according to necessity. Anaximenes in turn held that the arche was air, although John Burnet argues that by this, he meant that it was a transparent mist, the aether. Despite their varied answers, the Milesian school was searching for a natural substance that would remain unchanged despite appearing in different forms, and thus represents one of the first scientific attempts to answer the question that would lead to the development of modern atomic theory; "the Milesians," says Burnet, "asked for the φύσις of all things."