Turning Point. The early fifth century is a turning point in the history of Greek science, since it was then that the problem of change first began to be raised in a systematic manner. The Milesians had focused their attention mainly on identifying the basic material of the world; and the Pythagoreans turned away from that material element to look instead at the mathematical structure and form of the world. Neither group directly addressed a simple but crucial fact of everyday observation: things change.

The Cycle of Air. Change is an obvious feature of all things and is directly confirmed by the senses. But what exactly is it? And how does the fact that the world always changes impact on the ability to understand it scientifically? Anaximenes more than any other earlier thinker seems to have speculated on how...