Full Idea
Unlike Anaximander, Anaximenes' underlying nature is not boundless, but specific, since he says that it is air, and claims that it is thanks to rarefaction and condensation that it manifests in different forms in different things.
Gist of Idea
For Anaximenes nature is air, which takes different forms by rarefaction and condensation
Source
report of Anaximenes (fragments/reports [c.546 BCE], A5) by Simplicius - On Aristotle's 'Physics' 9.24.26-
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.17