Book 3
23
because obviously not only will there be lines apart from both Ideal and sensible lines, but it will be the same with each of the other classes.sc. of objects of mathematical sciences. Thus since astronomy is one of the mathematical sciences, there will have to be a heaven besides the sensible heaven, and a sun and moon, and all the other heavenly bodies.
24
But how are we to believe this? Nor is it reasonable that the heaven should be immovable; but that it should move is utterly impossible.The reference is to the supposed intermediate heaven. A heaven (including heavenly bodies) without motion is unthinkable; but a non-sensible heaven can have no motion. It is the same with the objects of optics and the mathematical theory of harmony; these too, for the same reasons, cannot exist apart from sensible objects. Because if there are intermediate objects of sense and sensations, clearly there will also be animals intermediate between the Ideal animals and the perishable animals.If there are intermediate, i.e. non-sensible, sights and sounds, there must be intermediate faculties of sight and hearing, and intermediate animals to exercise these faculties; which is absurd.
25
One might also raise the question with respect to what kind of objects we are to look for these sciences. For if we are to take it that the only difference between mensuration and geometry is that the one is concerned with things which we can perceive and the other with things which we cannot, clearly there will be a science parallel to medicine (and to each of the other sciences), intermediate between Ideal medicine and the medicine which we know.
