Anaximenes of Miletus

Life and Work
Anaximenes (fl. c.545 BC) was a discipline of Anaximander. He is the third and the last of the Milesian philosophers. Only a few sources survive for his life and activities. He wrote a book in prose probably within the same framework of natural philosophy as that of Anaximander. Anaximenes speculated on cosmology, cosmogony and meteorology.
The Air
For Anaximenes, in contrast to Anaximander, the source of all things is not an indefinite and unlimited apeiron but the air (aer): a definite material substance. The air by the process of ‘rarefaction’ becomes fire and by the process of ‘condensation’ becomes water and earth. Hot and cold do not have an ontological or material status but they are due to rarefaction and condensation. For Anaximenes the earth is flat and rides on a cushion of air. A heavenly firmament revolves like a felt cap around it. The heavenly bodies were made by rarefaction into fire, they are also flat and rest on air.
The Soul
For Anaximenes, the air is divine and causes life. It is also the source of life which encloses the cosmos as well as the first principle that is responsible for the maintenance of all living organisms. The air is the divine psychic principle between microcosm and macrocosm. As the soul (air) of an individual organism maintains the single individual organism, so the soul of the cosmos (universal breath) surrounds and maintains the whole universe. Hence Anaximenes’ cosmos is conceived as a huge animate being with divine origins.
Fragments and Testimonies
1 (A5 from Theophrastus) Anaximenes, Anaximander's colleague, said, as he did, that there was one underlying nature, but not, as he did, that it was limitless but limited, naming it as air; and by thinning and thickening it makes individual objects different.
2(2) As our soul, which is air, maintains us, so breath and air surround the whole world.