The third physical philosopher produced by Miletus, seemingly before the time of her terrible disasters suffered from the Persians after the Ionic revolt between 500-494 B.C., was Anaximenes, who struck out a third hypothesis. He assumed, as the primordial substance, and as the source of all generation or transmutation, Air, eternal in duration, infinite in extent. He thus returned to the principle of the Thaletian theory, selecting for his beginning a known substance, though not the same substance as Thales. To explain how generation of new products was possible (as Anaximander had tried to explain by his theory of evolution of latent contraries), Anaximenes adverted to the facts of condensation and rarefaction, which he connected respectively with cold and heat.[17] The Infinite Air, possessing and exercising an inherent generative and developing power, perpetually in motion, passing from dense to rare or from rare to dense, became in its utmost rarefaction, Fire and Æther; when passing through successive stages of increased condensation it became first cloud, next water, then earth, and, lastly, in its utmost density, stone.[18] Surrounding, embracing, and pervading the Kosmos, it also embodied and carried with it a vital principle, which animals obtained from it by inspiration, and which they lost as soon as they ceased to breathe.[19] Anaximenes included in his treatise (which was written in a clear Ionic dialect) many speculations on astronomy and meteorology, differing widely from those of Anaximander. He conceived the Earth as a broad, flat, round plate, resting on the air.[20] Earth, Sun, and Moon were in his view condensed air, the Sun acquiring heat by the extreme and incessant velocity with which he moved. The Heaven was not an entire hollow sphere encompassing the Earth below as well as above, but a hemisphere covering the Earth above, and revolving laterally round it like a cap round the head.[21] [Footnote 17: Origen. Philosophumen. c. 7; Simplikius in Aristot. Physic. f. 32; Brandis, Handb. d. Gesch. d. Gr.-R. Phil. p. 144. Cicero, Academic. ii. 37, 118. "Anaximenes infinitum aera, sed ea, quæ ex eo orirentur, definita." The comic poet Philemon introduced in one of his dramas, of which a short fragment is preserved (Frag. 2, Meineke, p. 840), the omnipresent and omniscient Air, to deliver the prologue: [Greek: ----ou(to/s ei)m' e)gô\ A)ê/r, o(\n a)/n tis o)noma/seie kai\ Di/a. e)gô\ d', o(\ theou=' stin e)/rgon, ei)mi\ pantachou=-- pa/nt' e)x a)na/gkês oi)=da, pantachou= parô/n.]] [Footnote 18: Plutarch, De Primo Frigido, p. 917; Plutarch, ap. Euseb. P. E. i. 8.] [Footnote 19: Plutarch, Placit. Philosophor, i. 3, p. 878.] [Footnote 20: Aristotel. De Coelo, ii. 13; Plutarch, Placit. Philosoph. iii. 10, p. 895.] [Footnote 21: Origen. Philosophum. p. 12, ed. Miller: [Greek: ô(sperei\ peri\ tê\n ê(mete/ran kephalê\n stre/phetai to\ pili/on.]]