The next noteworthy early Greek philosopher, Anaximenes, was an associate and follower of Anaximander. But if I were Anaximander, I would be pretty mad, accusing Anaximenes of ruining my metaphysics. Here is what one writer reports of Anaximenes:

Anaximenes of Miletus, son of Eurystratus, and associate of Anaximander, also says that the underlying nature of things is one and infinite. But he does not regard it as indeterminate, as Anaximander does, but as determinate, calling it air. And he says that it differs in respect of thinness in different things. When dilated it becomes fire; when compressed, wind and then cloud. When it is compressed further it becomes water, then earth then stone. The rest are produced from these. He too makes motion eternal, through which for him change also comes about. (Simplicus, Physis, 24, 26)

This passage describes Anaximenes as denying that the eternal source of reality is a principle beyond the visible world, which is Anaximander’s principle metaphysical claim. Instead, Anaximenes says, recalling Thales, that the source of all reality is one of the elements, air. When air is rarified, it becomes fire (another element); when condensed, it becomes water and then earth and stone (the other elements). So, he provides an explanation in terms of what we would call natural science of all the basic elements of nature, all the while denying or finding no need for anything beyond nature (meta-physical). He thinks he has a complete system, as it were, without any such recourse.