hollow sphere concentric with the earth. Next comes the hollow sphere of air concentric with the other two. Its natural motion when away from its place in the direction of the earth is in a straight line toward the circumference of the world, not however going beyond the sphere of the lightest element of all, namely, fire. This has its natural place outside of the other elements, also in the form of a hollow sphere concentric with the other three. Its natural motion is in a straight line away from the centre of the world and in the direction of the circumference. Our earth, water, air and fire are not really the elements in their purity. Each one has in it also mixtures of the other three elements, the one which gives it the name predominating.
All minerals, plants and animals are formed from these four elements by various combinations, all together forming the sublunar world, or the world of generation and decay. No individual thing in this world is permanent. All are subject to change and to ultimate destruction, though the destruction of one thing is the genesis of another. There is no annihilation.
The causes of the various combinations of the elements and the generation and destruction of mineral, plant and animal resulting therefrom, are the motions of the heavenly bodies. These are made of a purer substance than that of the four elements, the ether. This is proven by the fact that the heavenly bodies are not subject to change or destruction. They are all permanent and the only change visible in them is change of place. But even their motions are different from those of the four elements. The latter are in a straight line toward the centre or away from it, whereas the heavenly bodies move in a circle eternally around the centre. This is another proof that they are not composed of the same material as sublunar bodies.
The heavens consist of transparent spheres, and the stars as well as the planets are set in them and remain fixed. The motions of the heavenly bodies are due to the revolutions of the spheres in which they are set. These spheres are hollow and concentric. The outer-most sphere forming the outer limit of the universe (the world is finite according to Aristotle) is studded with the fixed stars and moves from east to west, making a complete revolution in twenty-four hours. This motion is transmitted to the other spheres which carry
