One ancient Greek said everything in existence is just condensed or thinned air

In the 6th century BC, Anaximenes of Miletus argued that air, not water or fire, was the single underlying substance of reality — compressed it became wind, cloud, water and eventually stone; thinned, it became fire. It was one of the earliest attempts to explain the entire physical world through one natural process rather than myth.

— Anaximenes of Miletus, Fragments of Anaximenes — 6th century BC, surviving only in later quotations