These, then, are the views which have been advanced by others and the terms in which they state them.We may begin our own statement by settling a question which to some has been the main difficulty-the question why some bodies move always and naturally upward and others downward, while others again move both upward and downward.After that we will inquire into light and heavy and of the various phenomena connected with them.The local movement of each body into its own place must be regarded as similar to what happens in connexion with other forms of generation and change.There are, in fact, three kinds of movement, affecting respectively the size, the form, and the place of a thing, and in each it is observable that change proceeds from a contrary to a contrary or to something intermediate: it is never the change of any chance subject in any chance direction, nor, similarly, is the relation of the mover to its object fortuitous: the thing altered is different from the thing increased, and precisely the same difference holds between that which produces alteration and that which produces increase.In the same manner it must be thought that produces local motion and that which is so moved are not fortuitously related.Now, that which produces upward and downward movement is that which produces weight and lightness, and that which is moved is that which is potentially heavy or light, and the movement of each body to its own place is motion towards its own form.(It is best to interpret in this sense the common statement of the older writers that 'like moves to like'.
For the words are not in every sense true to fact.If one were to remove the earth to where the moon now is, the various fragments of earth would each move not towards it but to the place in which it now is.In general, when a number of similar and undifferentiated bodies are moved with the same motion this result is necessarily produced, viz.that the place which is the natural goal of the movement of each single part is also that of the whole.But since the place of a thing is the boundary of that which contains it, and the continent of all things that move upward or downward is the extremity and the centre, and this boundary comes to be, in a sense, the form of that which is contained, it is to its like that a body moves when it moves to its own place.For the successive members of the scries are like one another: water, I mean, is like air and air like fire, and between intermediates the relation may be converted, though not between them and the extremes; thus air is like water, but water is like earth: for the relation of each outer body to that which is next within it is that of form to matter.) Thus to ask why fire moves upward and earth downward is the same as to ask why the healable, when moved and changed qua healable, attains health and not whiteness; and similar questions might be asked concerning any other subject of aletion.Of course the subject of increase, when changed qua increasable, attains not health but a superior size.The same applies in the other cases.One thing changes in quality, another in quantity: and so in place, a light thing goes upward, a heavy thing downward.The only difference is that in the last case, viz.that of the heavy and the light, the bodies are thought to have a spring of change within themselves, while the subjects of healing and increase are thought to be moved purely 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.
