Anaximenes was the third and last member (the others were Thales and Anaximander) of what is traditionally called the Milesian school of natural philosophers (physiologoi). The date of his death is estimated 528/526 BCE; it is probable that he "flourished" about 545 BCE. Although little is known about his life and work, fragments of ancient testimony credit him with studies under his older contemporary, Anaximander; with the writing of a book in "simple Ionic"; and with the doctrine that air is the underlying principle of the universe, changes in physical state being the result of its condensation and rarefaction. It is likely that Aristotle read Anaximenes' book and that Theophrastus had access to it. Several of the doxographers (Aëtius, Hippolytus, Diogenes Laertius) may have read later Hellenistic versions of the work.

On the strength of ancient testimony, historians of philosophy after Aristotle regarded...