Anaximenes proposed air as an indefinite, living principle and explained visible differences through rarefaction and condensation, making qualitative change depend on a continuous physical process.
Intellectual setting
Later writers group Anaximenes with the Milesian search for an explanatory first principle, though the surviving evidence is indirect and selective.
Key ideas
Air as archê
Air is presented as the underlying stuff from which the familiar varieties of nature arise.
Rarefaction and condensation
Density changes explain fire, wind, cloud, water, earth, and stones without introducing separate elemental origins.
Legacy
His account became a standard case in histories of early Greek natural explanation because it connects a single substrate to observable transformation.