Life
Anaximenes in their own century
Anaximenes was the last of the three Milesian thinkers and is said to have been a companion of Anaximander. His book is lost; what survives are brief reports of his cosmology and a few quoted phrases.
He wrote in the years before Persia absorbed Ionia, when Miletus was still prosperous enough for its citizens to argue about first principles in prose rather than verse.
Central ideas
Air as the origin
Air is the one thing everything else is made from: invisible, plainly there, and never at rest.
Condensation and rarefaction
Thinned, air becomes fire; thickened, it becomes wind, cloud, water, earth and stone. This is the first Greek attempt to say by what process one thing turns into another.
The cosmos as breath
He compared the air that holds the world together to the breath that holds a living body together, a comparison the Stoics later took up.
Criticism & debate
His cosmology was superseded quickly, and ancient writers treated him as a step towards better theories rather than reading him closely, so the reports are thin.
Influence
Explaining difference in kind by difference in degree gave later thinkers a model they kept reaching for, from the atomists to early modern physics.
How to read them
Begin Anaximenes with the tension between air as the origin and condensation and rarefaction. Read the major works slowly enough to notice how biography, public crisis and method shape the argument rather than treating the thinker as a list of doctrines.
If Anaximenes is right about air as the origin, what habit of judgement would have to change first?