Anaximenes ... also says that the underlying nature is one...

Anaximenes ... also says that the underlying nature is one and infinite ... but not undefined as Anaximander said but definite, for he identifies it as air; and it differs in its substantial nature by rarity and density. Being made finer it becomes fire; being made thicker it becomes wind, then cloud, then (when thickened still more) water, then earth, then stones; and the rest come into being from these.