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 primary substance of which all other things are made. 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 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.
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