He speaks clearly of the four Empedoklean elements (fr. 2), and he is careful to attribute to Air the attributes of Nous as taught by Anaxagoras (fr. 4.). The doxographical tradition as to his cosmological views is fairly preserved:
Diogenes of Apollonia makes air the element, and holds that all things are in motion, and that there are innumerable worlds. And he describes the origin of the world thus. When the All moves and becomes rare in one place and dense in another, where the dense met together it formed a mass, and then the other things arose in the same way, the lightest parts occupying the highest position and producing the sun. [Plut.] Strom. fr. 12 (R. P. 215).
Nothing arises from what is not nor passes away into what is not. The earth is round, poised in the middle, having received its shape through the revolution proceeding from the warm and its solidification from the cold. Diog. ix. 57 (R. P. 215).
The heavenly bodies were like pumice-stone. He thinks they are the breathing-holes of the world, and that they are red-hot. Aet. ii. 13, 5 = Stob. i. 508 (R. P. 215).
The sun was like pumice-stone, and into it the rays from the aether fix themselves. Aet. ii. 20, 10. The moon was a pumicelike conflagration. Ib. ii. 25, 10.
Along with the visible heavenly bodies revolve invisible stones, which for that very reason are nameless; but they often fall and are extinguished on the earth like the stone star which fell down flaming at Aigospotamos.[1] Ib. ii. 13, 9.
We have here nothing more than the old Ionian doctrine with a few additions from more recent sources. Rarefaction and condensation still hold their place in the explanation of the opposites, warm and cold, dry and moist, stable and mobile (fr. 5). The differentiations into opposites which Air may undergo are, as Anaxagoras had taught, infinite in number; but all may be reduced to the primary opposition of rare and dense. We may gather, too, from Censorinus,[2] that Diogenes did not, like Anaximenes, speak of earth and water as arising from Air by condensation, but rather of blood,
