Anaximenes was the Man
Anaximenes is the third of the Milesian philosophers who actually seem to have invented philosophy from scratch, in that they sought naturalist explanations rather than supernatural ones. As with his predecessors, Thales and Anaximander, most of what we know about his thought comes from later philosophers, like Aristotle, who criticised it.
Anaximenes is most famous for believing that the basic substance was air, and that everything else was made up of it through condensation. At first thought, I found this silly, but on second thought, not so much. Suppose a modern time traveling cosmologist found himself in ancient Miletus, and wanted to explain his science to his fellow citizens.
What might he tell them? Maybe something like "In the beginning there was a hot dense energy plasma." Of course nobody would know what energy or plasma was, so he might try to cast his discussion in terms of something more familiar, like air. His fellows likely knew that water existed in liquid and solid forms, as did various metals and even the rock spewed by volcanoes. They could reasonably infer that water, at least, existed in an air-like vapor form.
But how about you, me, and that there tree? We too, and the tree, are made mostly of air: water, carbon, nitrogen and oxygen. Even bones and stones were vapor once, or a sort of air, in a star or in the pre-solar nebula.
Anaximenes was reputedly a monist, desiring to explain everything in a one substance creates all formulation. Can we reconcile the mixed elemental compositions of Earth and air in such terms? Well of course! In the beginning, the air-like primordial plasma knew no atoms. They were formed by recombination, a kind of condensation, in the early universe. Other elements were formed by nuclear fusion, yet another condensation process, in stars.