Because the composition of the things formed of that substance varies. For the common substance at first received four different forms, and each form was endowed with two qualities, and through these four qualities the substance was turned into the elements of which all things are formed. The composition of the elements takes place in the following manner:-First they are mixed in consequence of the motion of the spheres, and then they combine together; a cause for variation arises then in the variation of the degree of heat, cold, moisture, and dryness of the elements which form the constituent parts of the things. By these different combinations things are variously predisposed to receive different forms: and these in their turn are again prepared to receive other forms, and so on. Each generic form finds a wide sphere in its substance both as regards quality and quantity: and the individuals of the classes vary accordingly. This is fully explained in Natural Science. It is quite correct and clear to every one that readily acknowledges the truth, and does not wish to deceive himself.
We. -- Since the combination of the elements prepares substances and enables them to receive different forms, what has prepared the first substance and caused one part of it to receive the form of fire, another part the form of earth, and the parts between these two the forms of water and of air, since one substance is common to all? Through what has the substance of earth become more fit for the form of earth, and the substance of fire more fit for that of fire?
Ar. -- The difference of the elements was caused by their different position for the different places prepared the same substance differently, in the following way: the portion nearest the surrounding sphere became more rarified and swifter in motion, and thus approaching the nature of that sphere, it received by this preparation the form of fire. The farther the substance is away from the surrounding sphere towards the centre, the denser, the more solid, and the less luminous it is; it becomes earth; the same is the cause of the formation of water and air. This is necessarily so: for it would be absurd to deny that each part of the substance is in a certain place: or to assume that the surface is identical with the centre, or the centre with the surface. This difference in place determined the different forms, i.e., predisposed the substance to receive different forms.
We. -- Is the substance of the surrounding sphere, i.e., the heavens, the same as that of the elements?
Ar. -- No; the substance is different, and the forms are different. The term" body" is homonymously used of these bodies below and of the heavens, as has been shown by modern philosophers. All this has been demonstrated by proof.
But let now the reader of this treatise hear what I have to say. Aristotle bass proved that the difference of forms becomes evident by the difference of actions. Since, therefore, the motion of the elements is rectilinear, and that of the spheres circular, we infer that the substances are different. This inference is supported by Natural Science. When we further notice that substances with rectilinear motion differ in their directions, that some move upward, some downward, and that substances which move in the same direction have different velocities, we infer that their forms must be different. Thus
