In order to find premisses for our argument, we must answer the
question, What is that which, in natural objects, makes them easily
destroyed, or the reverse? Since fire and water, and whatsoever is
akin thereto, do not possess identical powers they are reciprocal
causes of generation and decay. Hence it is natural to infer that
everything else arising from them and composed of them should share in
the same nature, in all cases where things are not, like a house, a
composite unity formed by the synthesis of many things.

    In other matters a different account must be given; for in many
things their mode of dissolution is something peculiar to
themselves, e.g. in knowledge and health and disease. These pass
away even though the medium in which they are found is not destroyed
but continues to exist; for example, take the termination of
ignorance, which is recollection or learning, while knowledge passes
away into forgetfulness, or error. But accidentally the disintegration
of a natural object is accompanied by the destruction of the
non-physical reality; for, when the animal dies, the health or
knowledge resident in it passes away too. Hence from these
considerations we may draw a conclusion about the soul too; for, if
the inherence of soul in body is not a matter of nature but like
that of knowledge in the soul, there would be another mode of
dissolution pertaining to it besides that which occurs when the body
is destroyed. But since evidently it does not admit of this dual
dissolution, the soul must stand in a different case in respect of its
union with the body.