What further proves that movement is a genus, is that it is very
difficult, if not impossible, to grasp it by a definition. But how can
it be called a form when its result is deterioration, or something
passive? It may then be compared to the warming influence of the rays
of the sun, which exerts on some things an influence that makes them
grow, while other things it shrivels. In both cases, the movement has
something in common, and is identical, so far as it is a movement; the
difference of its results is due to the difference of the beings in
which it operates. Are then growing sick and convalescence identical?
Yes, so far as they are movements. Is their difference then due to
their subjects, or to anything else? This question we will consider
further on, while studying alteration. Now let us examine the elements
common to all movements; in that way we shall be able to prove that
movement is a genus.