Anaximenes in Harpers Dictionary of Classical Antiquities
A Greek philosopher of Miletus, a younger contemporary and pupil of Anaximander, who died about B.C. 502. He supposed air to be the fundamental principle, out of which everything arose by rarefaction and condensation. This doctrine he expounded in a work, now lost, written in the Ionic dialect.
Anaximenes of Miletus in Wikipedia
Anaximenes (Greek: Άναξιμένης) of Miletus (fl. 585 BCE, d. 528 BCE) was an Archaic Greek Pre-Socratic philosopher also known as the father of philosophy active in the latter half of the 6th century BC. One of the three Milesian philosophers, he is identified as a younger friend or student of Anaximander. Anaximenes, like others in his school of thought, practiced material monism. This tendency to identify one specific underlying reality made up of a material thing constitutes the bulk of the contributions for which Anaximenes is most famed.
Anaximenes and the Arche
While his predecessors Thales and Anaximander proposed that the arche, the underlying material of the world, were water and the ambiguous substance apeiron, respectively, Anaximenes asserted that air was this most basic stuff of which all other things are made. To him air was not just one of the four basic elements but was the substance from which all things came and to which they would all eventually return. While the choice of air may seem arbitrary, he based his conclusion on naturally observable phenomena in the process of rarefaction and condensation. When air condenses it becomes visible, as mist and then rain and other forms of precipitation, and as the condensed air cools Anixemenes supposed that it went on to form earth and ultimately stones. In contrast, water evaporates into air which ignites and produces flame when further rarefied. While other philosophers also recognized such transitions in states of matter, Anaximenes was the 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.
The Origin of the Cosmos
Having concluded that everything in the world is composed of air, Anaximenes then used his theory to devise a scheme explaining the origins and nature of the earth as well as of the surrounding celestial bodies. Air felted to create the flat disk of the earth, which he said was table-like and behaved like a leaf floating on air. In keeping with the prevailing view of celestial bodies as balls of fire in the sky, Anaximenes proposed that the earth let out an exhalation of air that rarefied, ignited and became the stars. While the sun is similarly described as being aflame, it is not composed of rarefied air like the stars but rather of earth like the moon; its burning comes not from its composition but rather from its rapid motion. The moon and sun are likewise considered to be flat and floating on streams of air, and when the sun sets it does not pass under the earth but is merely obscured by higher parts of the earth as it circles around and becomes more distant; the motion of the sun and the other celestial bodies around the earth is likened by Anaximenes to the way that a cap may be turned around the head.
Anaximenes used his observations and reasoning to provide causes for other natural phenomena on the earth as well. Earthquakes he asserted were the result either of lack of moisture, which causes the earth to break apart because of how parched it is, or of overabundance thereof, which also causes cracks in the earth because of the excess of water. In either case the earth becomes weakened by its cracks and hills collapse, causing earthquakes. Lightning is also caused by a violent separation, this time of clouds by winds to create a bright, fire-like flash. Rainbows are formed when densely compressed air is touched by the rays of the sun. These examples further show how Anaximenes like the other Milesians looked for the broader picture in nature, seeking unifying causes for diversely occurring events rather than treating each one on a case-by-case basis or attributing them to gods or a personified nature.