Diogenes of Apollonia (dīŏj`ənēz ăpəlō`nēə), 5th cent. B.C., Greek philosopher. An eclectic, he reverted to the Milesian tradition of a century earlier in seeking to explain the constitution of all matter in terms of a single basic stuff. He believed, with Anaximenes, that this substance was air and, with Anaxagoras, that a principle of intelligence, or Nous, was responsible for governing and diffusing air. Some of Diogenes' extensive work in physiology was preserved by Aristotle.