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: