Anaximenes of Miletus Greek philosopher flourished c. 545 bc Main Greek philosopher of nature and one of three thinkers of Miletus traditionally considered to be the first philosophers in the Western world. Of the other two, Thales held that water is the basic building block of all matter, whereas Anaximander chose to call the essential substance “the unlimited.” Anaximenes substituted aer (“mist,” “vapour,” “air”) for his predecessors’ choices. His writings, which survived into the Hellenistic Age, no longer exist except in passages in the works of later authors. Consequently, interpretations of his beliefs are frequently in conflict. It is clear, however, that he believed in degrees of condensation of moisture that corresponded to the densities of various types of matter. When “most evenly distributed,” aer is the common, invisible air of the atmosphere. By condensation it becomes visible, first as mist or cloud, then as water, and finally as solid matter such as earth or stones. If further rarefied, it turns to fire. Thus hotness and dryness typify rarity, whereas coldness and wetness are related to denser matter. Anaximenes’ assumption that aer is everlastingly in motion suggests that he thought it also possessed life. Because it was eternally alive, aer took on qualities of the divine and became the cause of other gods as well as of all matter. The same motion accounts for the shift from one physical state of the aer to another. There is evidence that he made the common analogy between the divine air that sustains the universe and the human “air,” or soul, that animates people. Such a comparison between a macrocosm and a microcosm would also permit him to maintain a unity behind diversity as well as to reinforce the view of his contemporaries that there is an overarching principle regulating all life and behaviour. A practical man and a talented observer with a vivid imagination, Anaximenes noted the rainbows occasionally seen in moonlight and described the phosphorescent glow given off by an oar blade breaking the water. His thought is typical of the transition from mythology to science; its rationality is evident from his discussion of the rainbow not as a goddess but as the effect of sun rays on compacted air. Yet his thought is not completely liberated from earlier mythological or mystical tendencies, as seen from his belief that the universe is hemispherical. Thus, his permanent contribution lies not in his cosmology but in his suggestion that known natural processes (i.e., condensation and rarefaction) play a part in the making of a world. This suggestion, together with Anaximenes’ reduction of apparent qualitative differences in substances to mere differences of quantity, was highly influential in the development of scientific thought.