The Greek philosopher Anaximenes (active 546 B.C.), last of the important philosophers of Miletus, was perhaps the first philosopher to insist on an underlying physical law governing the universe.
The details of the life of Anaximenes are almost totally unknown, but he is said to have flourished in the year of the fall of Sardis. He wrote at least one work expounding his philosophical views, and although now lost it probably survived into Hellenistic times. What is known of Anaximenes's views emerges largely from discussion and criticism of his work by Aristotle and others. There is no question that Anaximenes was familiar with Anaximander's writings, since their views are very close.
Anaximenes postulated aer, meaning "vapor" or "air," as the basic substance out of which all other things arise. He described it as being invisible when evenly distributed, but by the process of condensation it becomes visible as cloud, water, and finally earth and stone. Rarefaction, on the other hand, causes air to expand and become hot and then turn to fire. Thus, Anaximenes could explain the creation of all forms of matter through the mechanism of condensation and rarefaction of this substance, air, that is obviously composed of discrete particles.
Anaximenes also assumed the air to be in a state of perpetual motion. This provided an explanation for the changes of density which produced the infinite number of worlds that came into being and then disappeared, being reabsorbed into the infinite air. He equated the air that supports the universe with human breath, which is identified with the soul. This implication that air possessed life was compatible with the contemporary belief in the identification of air or breath with life.
In his cosmology Anaximenes describes the earth, the first heavenly body to take shape, as having come into being through condensation; it is flat and floats, as do all heavenly bodies, on the primal and indefinitely extended air. The other heavenly bodies are fire in substance and arose by rarefaction of the water given off by earth. Anaximenes went on to describe the universe not as a complete sphere like Anaximander's but as hemispherical, with the stars passing around, not under, the earth.
In his attempt to present a rational, scientific view, in the form of describing a natural process as responsible for making a world, and by reducing qualitative differences to quantitative differences, Anaximenes was only partially free from mythological beliefs. However, he provided a pattern to be followed by the natural philosophers in the development of science.
There is no full-length biography of Anaximenes, but G.S. Kirk and J.E. Raven, The Presocratic Philosophers (1962), gives a good account of his life and work, although it is somewhat difficult to read. Cyril Bailey, The Greek Atomists and Epicures (1964), is a very readable exposition in which the ideas of Anaximenes are clarified and placed in historical perspective. See also W.K.C. Guthrie, A History of Greek Philosophy (2 vols., 1962), and Felix M. Cleve, The Giants of Pre-Socratic Greek Philosophy (2 vols., 1965).