Anaximenes of Miletus

Anaximenes (b. 585 BC - d. 528 BC, ~57 years old, lived during the Archaic period (c. 750 – c. 500 BC)) was a pre-Socratic Greek philosopher who lived in Miletus, Ionia in Asia Minor (modern west coast turkey). As one of the three Milesian philosophers, he is identified as a younger friend or student of Anaximander. 

• Material Monism - all of the world's objects (or universe) are composed of a single element (the source of everything), which for Anaximenes was air, the first principle, the primary substance.
• First to associate the quality pairs hot/dry and cold/wet with the density of a single material and add a quantitative dimension to the Milesian monistic system
• First recorded thinker who provided a theory of change (by the processes of rarefaction and condensation), and supported it with logic and observation
• By providing cosmological accounts with a theory of change, he separated them from the realm of mere speculation and made them, at least in conception, scientific theories capable of testing