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On The Heavens   

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,

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