Let us, indeed, suppose, that something which formerly was a potentiality succeeds in assuming a form, as "potentiality that becomes a statue," or that passes to actualization, as a man's walk.[421] In the case where the metal becomes a statue, this passage is a movement; in the case of the walking, the walk itself is a movement, like the dance, with one who is capable of it. In the movement of the first kind, where the metal passes into the condition of being a statue, there is the production of another form which is realized by the movement.[422] The movement of the second kind, the dance, is a simple form of the potentiality, and, when it has ceased, leaves nothing that subsists after it.[423] MOVEMENT IS ACTIVE FORM, AND CAUSE OF OTHER FORMS. We are therefore justified in calling movement "an active form that is aroused," by opposition to the other forms which remain inactive. (They may be so named), whether or not they be permanent. We may add that it is "the cause of the other forms," when it results in producing something else. This (sense-) movement may also be called the "life of bodies." I say "this movement," because it bears the same name as the movements of the intelligence, and those of the soul. QUESTIONS ABOUT MOVEMENT. 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. COMMON ELEMENT IN GROWTH, INCREASE AND GENERATION. First, the word "movement" can be used in different senses, just as essence, when considered a genus. Further, as we have already said, all the movements by which one thing arrives at a natural state, or produces an action suitable to its nature, constitute so many species. Then, the movements by which one thing arrives at a state contrary to its nature, have to be considered as analogous to that to which they lead.