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On The Heavens   

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,

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