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On The Heavens   



that they are generated from one another.



7



We must, therefore, turn to the question, what is the manner of

their generation from one another? Is it as Empedocles and

Democritus say, or as those who resolve bodies into planes say, or

is there yet another possibility? (1) What the followers of Empedocles

do, though without observing it themselves, is to reduce the

generation of elements out of one another to an illusion. They make it

a process of excretion from a body of what was in it all the time-as

though generation required a vessel rather than a material-so that

it involves no change of anything. And even if this were accepted,

there are other implications equally unsatisfactory. We do not

expect a mass of matter to be made heavier by compression. But they

will be bound to maintain this, if they say that water is a body

present in air and excreted from air, since air becomes heavier when

it turns into water. Again, when the mixed body is divided, they can

show no reason why one of the constituents must by itself take up more

room than the body did: but when water turns into air, the room

occupied is increased. The fact is that the finer body takes up more

room, as is obvious in any case of transformation. As the liquid is

converted into vapour or air the vessel which contains it is often

burst because it does not contain room enough. Now, if there is no

void at all, and if, as those who take this view say, there is no

expansion of bodies, the impossibility of this is manifest: and if

there is void and expansion, there is no accounting for the fact

that the body which results from division cfpies of necessity a

greater space. It is inevitable, too, that generation of one out of

another should come to a stop, since a finite quantum cannot contain

an infinity of finite quanta. When earth produces water something is

taken away from the earth, for the process is one of excretion. The

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