Diogenes of Apollonia (/daɪˈɒdʒəˌniːz/; Greek: Διογένης ὁ Ἀπολλωνιάτης; fl. 425 BC) was an ancient Greek philosopher, and was a native of the Milesian colony Apollonia in Thrace. He lived for some time in Athens. His doctrines are known chiefly from Diogenes Laërtius and Simplicius. He believed air to be the one source of all being, and, as a primal force, to be intelligent. All other substances are derived from it by condensation and rarefaction. Aristotle has preserved a long passage by Diogenes concerning the organization of the blood vessels.