The third great Milesian thinker, Anaximenes (d.c.528 BC) may have been Anaximander’s pupil and associate, but he was not afraid to contradict his mentor. Where Anaximander Anaximander criticised Thales's identification of water as the underlying material of the universe and posited that the first principle was not a particular substance but an eternal boundlessness (the apeiron or the indefinite),
Anaximenes returned to the single substance hypothesis, opting for aer (which could be mist, vapour, or air) rather than water.
He based this notion on his observations that air can take on different appearances, and supposed that under suitable conditions it even becomes different types of substances. He thought that when rarefied air becomes fire, and when condensed it becomes, in turn, wind, clouds, water and earth. This idea may have come from reflection on the transformations of water, from liquid to solid and vapour, events in which different materials can be seen to be various forms of the same thing. It then becomes a question of which of those is the base form.
Thales went for water, Anaximenes for the stage before the liquid.
Where Anaximander went for an ethereal entity (the aperion), Anaximenes opted for a tangible state of existence, arguing that air that can evolve into other substances through condensation and rarefaction. In his version, air (Pneuma the “breath of life”) was infinite and divine.
In Anaximenes's cosmology, the flat, table-like earth floated, along with the other heavenly bodies, on the air like a leaf under a dome-like sky. Movement of the celestial bodies stems from the air’s constant motion, while the earth, at the centre of the cosmos, somehow manages to stay stationary atop the air beneath it with some unexplained force or current preventing it from falling.
While the details of his life are obscure and undocumented, some of his writings survived until the Hellenistic Age, before they disappeared. Much of his astronomical thought seems to stem from Anaximander’s, with alterations to make them comply with his views and observations. There is some evidence to suggest he may have been the first person to distinguish between planets and stars and he also came up with explanations for earthquakes, lightning and rainbows, seeking underlying causes for seemingly unrelated occurrences.
Sources:
Chambers Biographical Dictionary
W. A. Heidel, Hecataeus and Xenophanes;
Richard D. McKirahan, Philosophy Before Socrates
Wikipedia