A problem with Anaximander's boundless is that once it acquired bounds and became differentiated into the components of an existing cosmos, it was no longer boundless. Anaximenes, a student, friend, and younger contemporary of Anaximander argued that the problem of change was produced by the monist tradition, that is the tradition that all things arise from and return to one fundamental substrata. What he suggested was that there was one element which had the power to become all others, and that was air. As he put it, air could be condenser or rarer, hotter or colder, and still remain the same substance.

Anaximander's idea that the underlying substrata of the cosmos was a boundless that resulted from the fusion of opposites was in a sense arbitrary. There were no known examples in nature for him to point to. On the other hand the concept of air as the substrata was in sympathy with the poetic and religious explanations of the creation of the cosmos popular in his day. It was this arbitrary nature that Anaximenes stressed in his opposition to Anaximander's concept of the boundless. He said that to account for a world-order by natural causes one must show its origin to have been due to a process which can still be verified today. This verification, Anaximenes claimed, could be found in the concept of condensation and rarification. When air is dispensed finely, he maintained, it is invisible, but is made visible by the hot and cold, wet, and movement. If it were not in constant motion things could not change. When most finely dispersed, it is fire. In fact, later commentators said that when it was at its finest dispersion it was the divine fire, the boundless of Anaximander.

Through the process of condensation it becomes water and finally earth. It is easy to see how this approach met one of the most important assumptions that lay beneath the monist approach to the origin of the cosmos. That the source and fount of all being had to be in eternal perpetual motion. It is this eternal motion that was the cause of all of the changes taking place. That air is in constant perpetual motion can be seen in every sunbeam. Air as that which is in eternal perpetual motion respected also the age-old tradition that identified breath with life.

One logical consequence of these thoughts was that there is a close affinity between the divine or universal mind that actuates the perpetual motion and the evolution of all things, and the human soul. "The air within us," said Diogenes, "is a small portion of the god." This links Anaximenes with one of the most powerful sources of ancient Greek thought, the idea that the underlying substance of the cosmos is air, it is divine, and that it is the stuff of the human soul. Undoubtedly one of the most important implications of the philosophy of Anaximenes is that it made man partly divine. For Thales and Anaximander reason itself was a divine activity that men partakes of, but in this bold step Anaximenes made the soul of man divine in itself.

The Milesian universe was not a mechanical universe. It was animated by the gods within. That which becomes arises out of perpetual motion. It is eternal, it is divine. Divine intervention became what the world is, why it is. The living gods of Homer have been absorbed into the fabric of existence. Homer's gods were personal and they were cunning. The Milesian gods were rational, understandable, divine, power. Their divinity was ultimately knowable through reasoning. But the world of reasoning and the world of experience are very different worlds. The world of reasoning includes the imperceptible that is necessarily implicit in the perceptible. Experience teaches us that there are many things in the world around us. Reasoning teaches us that they are ephemeral, they arise out of and descend into the imperceptible. They are but patterns in the eternal.