There is in the properties of the spheres another circumstance that is opposed to the assumed laws of Nature; namely, if the substance of all spheres is the same, why does it not occur that the form of one sphere combines with the substance of another sphere, as is the case with things on earth, simply because their substance is fit [for such changes]? If the substance of all spheres is the same, if it is not assumed that each of them has a peculiar substance, and if, contrary to all principles, the peculiar motion of each sphere is no evidence for the special character of its substance, why then should a certain form constantly remain united with a certain substance? Again, if the stars have all one substance, by what are they distinguished from each other? is it by forms? or by accidents? Whichever be the case, the forms or the accidents would interchange, so that they would successively unite with every one of the stars, so long as their substance [being the same] admits the combinations [with every one of the forms or the accidents]. This shows that the term substance, when used of the spheres or the stars, does not mean the same as it signifies when used of the substance of earthly things, but is applied to the two synonymously. It further shows that every one of the bodies of the spheres has its own peculiar form of existence different from that of all other beings. Why then is circular motion common to all spheres, and why is the fixed position of the stars in their respective spheres common to all stars? If we, however, assume design and determination of a Creator, in accordance with His incomprehensible wisdom, all these difficulties disappear. They must arise when we consider the whole Universe, not as the result of free will, but as the result of fixed laws of Nature: a theory which, on the one hand, is not in harmony with the existing order of things, and does not offer for it a sufficient reason or argument; and, on the other hand, implies many and great improbabilities. For, according to this theory, God, whose perfection in every respect is recognised by all thinking persons, is in such a relation to the Universe that He cannot change anything; if He wished to make the wing of a fly longer, or to reduce the number of the legs of a worm by one, He could not accomplish it. According to Aristotle, He does not try such a thing, and it is wholly impossible for Him to desire any change in the existing order of things; if He could, it would not increase His perfection; it might, on the contrary, from some point of view, diminish it.