The expression corporeal world includes the celestial spheres and all which is under them. To be sure, the body of the sphere is different from the other bodies in matter and form and qualities. It consists of a fifth nature, different from the four elements. It is not cold, or it would move downward like earth and water. It is not warm, or it would move upward like air and fire. It is not wet, for it would then roll like the waves of the sea. Nor is it dry, for it would condense and not move at all. Not being any one of these qualities, which constitute our four elements, the sphere is not a composite of them either; for the simple is prior to the composite, and we cannot regard the elements of the sublunar world as prior and superior to the spheres. The sphere is neither light nor heavy. For light and heavy are relative terms. An object is heavy when out of its natural place, light when in its natural place. Thus a stone is heavy when it is away from the earth, which is its natural place, but is light when it comes to rest where it belongs. The sphere is never out of its place or in its place, as it moves constantly in a circle. Hence it is neither light nor heavy. Ibn Zaddik's definition of light and heavy as being relative, and dependent on the relation of the object to its natural place is peculiar, and would lead him to say that fire and air are also heavy when out of their natural place, which is outside of, and above earth and water. But this does not seem in consonance with the Aristotelian use of these terms. According to Aristotle an object is heavy if its tendency is to move to the centre of the world; it is light if it moves away from the centre to the circumference. Hence earth and water are heavy, fire and air are light. The natural place of a body or element is that to which it has a tendency to move, or in which it has a tendency to rest, when left to itself. Hence a body will always move to its natural place when away from it and under no restriction; and its heaviness or lightness does not change with its position. To continue, the sphere moves in a circle, the most perfect of all motions, having neither beginning nor end. It is more perfect than all bodies, and the knowledge of God is not hidden from it as it is hidden from us. Whatever moves in a circle must move around a body at rest; for if it moves around another moving body, this second body must have another body around which it moves, and this third body another, and so on _ad infinitum_, which is impossible. Hence the sphere moves around a body at rest. This is the earth.