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 outermost 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 the planets. Since, however, we notice in the sun, moon and the other planetary bodies motions in the contrary direction in addition to that from east to west, there must be other spheres having the motions apparent to us in the positions of the planets borne by them. Thus a given body like the sun or moon is set in more than one sphere, each of which has its own proper motion, and the star's apparent motion is the resultant of the several motions of its spheres. Without entering into further details concerning these motions, it will be sufficient for us to know that Aristotle counted in all fifty-five spheres. First came the sphere of the fixed stars, then in order the spheres of Saturn, Jupiter, Mars, Mercury, Venus, Sun, Moon. God himself sets the outer sphere in motion, or rather is the eternal cause of its motion, as the object of its desire; and in the same way each of the other motions has also its proper mover, likewise a pure form or spirit, which moves its sphere in the same incorporeal and unmoved manner as God. Thus we have in the supra-lunar world pure forms without matter in God and the spirits of the spheres, whereas in the sublunar world matter and form are inseparable. Neither is found separately without the other.