The Greek philosopher Anaximenes (active in 546 BC), the last of the important philosophers of Miletus, was perhaps the first philosopher to insist on a basic physical law governing the universe.
Anaximenes postulate aer, which means “vapor” or “air”, as the basic substance from which all other things are born. He described it as invisible when it is evenly distributed, but with the condensation process it becomes visible as cloud, water and finally earth and stone. Rarefaction, on the other hand, causes the air to expand and become warm and then turn into fire. Thus, Anassimene could explain the creation of all forms of matter through the mechanism of condensation and rarefaction of this substance, air, which is obviously composed of discrete particles.
Anassimene also assumed the air to be in a state of perpetual motion. This provided an explanation for the density changes that produced the infinite number of worlds that were born and then disappeared, reabsorbed into the infinite air. He equated the air that sustains the universe with the human breath, which identifies with the soul. This implication that air possessed life was compatible with contemporary belief in the identification of air or breath with life.
In his cosmology Anassimene describes the earth, the first celestial body to take shape, as if born by condensation; it is flat and floats, like all celestial bodies, on the primordial and indefinitely extended air. The other celestial bodies are fire in substance and arose by rarefaction of the water emitted from the earth. Anaximander continued to describe the universe not as a complete sphere like Anaximander’s, but as hemispheric, with the stars passing around, not below, the earth.
In his attempt to present a rational and scientific vision, describing a natural process as responsible for creating a world, and reducing qualitative differences to quantitative differences, Anassimene was only partially free from mythological beliefs. However, he provided a model that natural philosophers must follow in the development of science.
More readings on Anassimene
There is no complete biography of Anassimene, but G.S. Kirk and J.E. Raven, The Presocratic Philosophers (1962), provides a good account of his life and work, although it is a bit difficult to read. Cyril Bailey, The Greek Atomists and Epicureans (1964), is a very readable exposition in which Anassimene’s ideas are clarified and placed in a historical perspective. See also W.K.C. Guthrie, A History of Greek Philosophy (2 vol., 1962), and Felix M. Cleve, The Giants of Pre-Socratic Greek Philosophy (2 vol., 1965).