Living from the mid 6th to late 4th century BC, Anaximenes was a fellow student and successor to Anaximander in Miletus.
Very little is known about him, a little from the few works of his that survived, but mostly from a treatise by Theophrastus. Whereas Thales believed that water was the primary cosmic substance, and Heraklitos, fire, Anaximander believed that air was the primordial element and principle, the most mobile and ubiquitous element which can appear as water, earth, or fire and capable of taking on a particular form with specific qualities.
Anaximander believed that the creation of the earth came first in the universe, that it was wide in form and rested in the depths of the infinite, with vapors rising from it that became fire, some which in turn became stars of the same shape as the earth, yet floating in the air.
He noted that the moon's light came from the sun and also that both solar and lunar eclipses were natural events based on natural causes. He also believed that there were stars invisible to humans which caused lunar eclipses.