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.