On Nature
His most famous work was "On Nature" (Περὶ Φύσεως, "Peri Physeos"), some fragments of which are preserved, chiefly by Simplicius. Diogenes, like Anaximenes, believed air to be the one source of all being, and all other substances to be derived from it by condensation and rarefaction. This he modified by the theories of his contemporary Anaxagoras, and asserted that air, the primal force, was intelligent: