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 “Causation,” and the like. After these prolegomena to physics, he went on to treat of the universe in orderly sequence, beginning with the divinest part, the circumference of the whole, or outer heaven, which, according to his views, bounded the world, being composed of ether, a substance distinct from that of the four elements. This region was the sphere of the stars; and below it, in the Aristotelian system, was the planetary sphere, with the seven planets (the sun and moon being reckoned among the number) moving in it. Both stars and planets he seems to have regarded as conscious, happy beings, moving in fixed orbits, and inhabiting regions free from all change and chance; and these regions formed the subject of his treatise ‘On the Heavens.’ Next to this he is thought to have composed his work ‘On Generation and Corruption,’ in order to expound those principles of physical change (dependent on the hot, the cold, the wet, and the dry), which in the higher parts of the universe had no existence. This treatise formed the transition to the sublunary sphere, immediately round the earth, in which the meteors and comets moved, and which was characterised by incessant change, and by the passing of things into and out of existence, and which became the subject of his next treatise—the ‘Meteorologies.’ The last book of this work brings us down to the earth itself, and indeed beneath its surface, for it discusses, in a curious theory, the formation of rocks and metals.

From this point Aristotle would seem to have started afresh with his array of physiological treatises, the first written of which may very likely have been that ‘On