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takes place must either be incorporeal or possess body; and if it
has body, there will be two bodies in the same place at the same time,
viz. that which is coming to be and that which was previously there,
while if it is incorporeal, there must be an extra-corporeal void.
But we have already shown that this is impossible. But, on the other
hand, it is equally impossible that the elements should be generated
from some kind of body. That would involve a body distinct from the
elements and prior to them. But if this body possesses weight or lightness,
it will be one of the elements; and if it has no tendency to movement,
it will be an immovable or mathematical entity, and therefore not
in a place at all. A place in which a thing is at rest is a place
in which it might move, either by constraint, i.e. unnaturally, or
in the absence of constraint, i.e. naturally. If, then, it is in a
place and somewhere, it will be one of the elements; and if it is
not in a place, nothing can come from it, since that which comes into
being and that out of which it comes must needs be together. The elements
therefore cannot be generated from something incorporeal nor from
a body which is not an element, and the only remaining alternative
is that they are generated from one another.
Part 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 same thing happens again when the residue produces water. But
this can only go on for ever, if the finite body contains an infinity,
which is impossible. Therefore the generation of elements out of one
another will not always continue.
(2) We have now explained that the mutual transformations of the elements
cannot take place by means of excretion. The remaining alternative
is that they should be generated by changing into one another. And
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ON THE HEAVENS 54
this in one of two ways, either by change of shape, as the same wax
takes the shape both of a sphere and of a cube, or, as some assert,
by resolution into planes. (a) Generation by change of shape would
necessarily involve the assertion of atomic bodies. For if the particles
were divisible there would be a part of fire which was not fire and
a part of earth which was not earth, for the reason that not every
part of a pyramid is a pyramid nor of a cube a cube. But if (b) the
process is resolution into planes, the first difficulty is that the
elements cannot all be generated out of one another. This they are
obliged to assert, and do assert. It is absurd, because it is unreasonable
that one element alone should have no part in the transformations,
and also contrary to the observed data of sense, according to which
all alike change into one another. In fact their explanation of the
observations is not consistent with the observations. And the reason
is that their ultimate principles are wrongly assumed: they had certain
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