The Greek philosopher Anaximenes (active 546 BC), last of the important philosophers of Miletus, was perhaps the first philosopher to insist on an underlying physical law governing the universe.

The details of the life of Anaximenes are almost totally unknown, but he is said to have flourished in the year of the fall of Sardis. He wrote at least one work expounding his philosophical views, and although now lost it probably survived into Hellenistic times. What is known of Anaximenes's views emerges largely from discussion and criticism of his work by Aristotle and others. There is no question that Anaximenes was familiar with Anaximander's writings, since their views are very close.

Anaximenes postulated aer, meaning "vapor" or "air," as the basic substance out of which all other things arise. He described it as being invisible when evenly distributed, but by the process of condensation it becomes visible as cloud...