While his predecessors Thales and Anaximander proposed that the archai (singular: arche, meaning the underlying material of the world) were water and the ambiguous substance apeiron, respectively, Anaximenes asserted that air was this primary substance of which all other things are made. The choice of air may seem arbitrary, but Anaximenes based his conclusion on naturally observable phenomena in the processes of rarefaction and condensation. When air condenses it becomes visible, as mist and then rain and other forms of precipitation. As the condensed air cools Anaximenes 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.
Having concluded that everything in the world is composed of air, Anaximenes used his theory to devise a scheme that explains the origins and nature of the earth and 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. Similarly, he considered the moon and sun to be flat and floating on streams of air. In his theory, 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. Anaximenes likens the motion of the sun and the other celestial bodies around the earth 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 of water, which also causes cracks in the earth. In either case the earth becomes weakened by its cracks, so that hills collapse and cause earthquakes. Lightning is similarly caused by the violent separation of clouds by the wind, creating a bright, fire-like flash. Rainbows, on the other hand, are formed when densely compressed air is touched by the rays of the sun. These examples show how Anaximenes, like the other Milesian philosophers, looked for the broader picture in nature. They sought unifying causes for diversely occurring events, rather than treating each one on a case-by-case basis, or attributing them to gods or to a personified nature.