Milesian School

Ancient historians of philosophy, beginning with Aristotle himself, treated Thales of Miletus as the first person on record to ask what the entire natural world is fundamentally made of and to answer with a natural substance, water, rather than a story about the gods. Whatever the historical accuracy of the eclipse prediction attributed to him, later tradition made Thales a byword for the new kind of inquiry Miletus represented, one that assumed the world runs according to consistent, discoverable principles rather than divine whim.

His successor Anaximander pushed the inquiry further, arguing that the basic stuff of the world could not itself be one of the familiar elements like water, since each element is already a specific, limited thing locked in opposition to others, hot against cold, wet against dry. Anaximander instead proposed the apeiron, an unbounded and indefinite substance from which the familiar opposites separate out and into which, in a famous surviving fragment, they must eventually return as a kind of cosmic justice for their imbalance.

Anaximenes, the third Milesian, returned to a concrete substance, air, but explained the different forms of matter as air undergoing rarefaction into fire and progressive condensation into wind, cloud, water, earth and stone, offering the first known Greek mechanism, rather than a metaphor, for how one substance becomes many. None of the three left more than fragments, quoted and summarized centuries later by Aristotle and by the historian of philosophy Theophrastus.

The claim that Thales was the first philosopher is not something Thales said about himself. It is a judgment Aristotle made roughly two centuries later, in the opening book of the Metaphysics, where he surveys earlier thinkers as a way of setting up his own theory of the four causes and singles out Thales as the first to propose that a single material substance underlies all change, water in his case, making Thales the founding figure of what Aristotle calls inquiry into the material cause specifically.

That framing did more than describe the Milesians, it created the category later history has used to read them. Aristotle's student Theophrastus systematized the same material into the Opinions of the Physicists, a doxography, a structured record of what each earlier thinker held, organized around exactly the question Aristotle had asked, and it is chiefly through Theophrastus, quoted and summarized by still later compilers, that fragments of Anaximander and Anaximenes survive at all. The effect is that modern readers meet the Milesians already sorted into Aristotle's own categories, material monists answering a question about underlying substance, when the little that survives in their own words suggests broader concerns, Anaximander's apeiron doing ethical as well as physical work, that the inherited frame does not fully capture. Aristotle did not merely record the first chapter of philosophy's history, he wrote the table of contents everyone since has read the Milesians through.