Influence. Though exact details of Milesian speculation are often difficult to capture, its significance is clear and hard to overstate. It is especially important for the degree to which it embodies influential trends that later became more explicit in the history of Greek scientific thought. The Milesians were the first to distinguish consistently between what is natural and what is supernatural, and also to exploit that distinction. These two categories might well be familiar to us now, but for thinkers in the sixth century B.C.E., the difference between them was revolutionary. The theories of Thales, Anaximander, and Anaximenes are as remarkable for what they omit as for what they actually say. Against a background of mythical and religious accounts of the origin and nature of the world, they instead declared that valid explanations of events must be found in regular...