Anaximenes was a Greek philosopher of Miletus, a younger contemporary and pupil of Anaximander, who died about 502. He was not a great original genius like Anaximander, and in some respects his cosmology falls far short of his predecessor's. His title to remembrance is based on his discovery of the formula which for the first time made the Milesian theory coherent: of rarefaction and condensation. He regarded 'air' -- the air we breathe, but also that which thickens into mist and water -- as the primary form of body; it holds an intermediate stage between water and fire. Thus, his theory resembles that of Thales. On the other hand, he thought of this air as boundless and as containing an infinite number of worlds, in this respect following Anaximander. The solitary fragment quoted form his work shows that he was influenced by the analogy of the microcosm and the macrocosm. 'As our soul,' he says, 'which is air, holds us together, so do breath and air encompass the whole world.' The world is thought of as breathing or inhaling air from the boundless mass outside it. This air he spoke of as a 'god'.

It is maintained that the Milesian cosmology was based on the primitive and popular theory of 'the four elements'. However, the scientific conception of an 'element' did not exist at this date. We shall see later that this was due to Empedocles, and it is only the place that the old quaternion of Fire, Air, Earth, and Water occupied in his system (and afterwards in that of Aristotle) that has led to these being called 'the four elements'. It is an unfortunate confusion, but it is very difficult to avoid it, and we must continue to use the word 'element' in two senses which have very little to do with one another. The spirit of Ionian civilization had been thoroughly secular, and this was one of the causes that favored the rise of science. The Milesian school came to an end with the fall of Miletus in 494 BC, but 'The Philosophy of Anaximenes', as it was called, continued to be taught in other Ionian cities.

Excerpt from the Encyclopedia Britannica without permission.