ely from without.Sometimes, however, even they change of themselves, ie.in response to a slight external movement reach health or increase, as the case may be.And since the same thing which is healable is also receptive of disease, it depends on whether it is moved qua healable or qua liable to disease whether the motion is towards health or towards disease.But the reason why the heavy and the light appear more than these things to contain within themselves the source of their movements is that their matter is nearest to being.This is indicated by the fact that locomotion belongs to bodies only when isolated from other bodies, and is generated last of the several kinds of movement; in order of being then it will be first.Now whenever air comes into being out of water, light out of heavy, it goes to the upper place.It is forthwith light: becoming is at an end, and in that place it has being.Obviously, then, it is a potentiality, which, in its passage to actuality, comes into that place and quantity and quality which belong to its actuality.And the same fact explains why what is already actually fire or earth moves, when nothing obstructs it, towards its own place.For motion is equally immediate in the case of nutriment, when nothing hinders, and in the case of the thing healed, when nothing stays the healing.But the movement is also due to the original creative force and to that which removes the hindrance or off which the moving thing rebounded, as was explained in our opening discussions, where we tried to show how none of these things moves itself.The reason of the various motions of the various bodies, and the meaning of the motion of a body to its own place, have now been explained.
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