This difference between souls is caused principally by the constitution
of the bodies they animate; also by the moral habits, the activities,
the thoughts and behavior of these souls in earlier existence.
According to Plato[109] the choice of the souls' condition depends on
their anterior existence. On observing the nature of souls in general,
we find that Plato recognizes differences between them by saying that
some souls occupy the second or third ranks.[110] Now we have said that
all souls are (potentially) all things,[111] that each is characterized
by the faculty principally exercised thereby, that is, that some souls
unite with the intelligible world by actualization, while others do so
in thought or desire.[112] Souls, thus contemplating different objects,
are and become all that they contemplate. Fulness and perfection
also belong to soul, but in this respect they are not all identical,
because variety is the law that directs their co-ordination. Indeed,
the universal[113] reason is on the one hand manifold, and on the other
varied, like a being that is animate, and which possesses manifold
forms.[114] In this case, there is co-ordination; beings are not
entirely separated from each other, and there is no place for chance
either in real beings, nor in bodies; consequently the number of beings
is definite. To be individual, beings must first be stable, then they
must remain identical, and last, they must numerically be one in order
to achieve individuality. Bodies which by nature perpetually ooze away,
because for them form is something incidental, never possess formal
existence but by their participation in (and imitation of), genuine
"Beings." On the contrary, for the latter, that are not composite,
existence consists in each of them being numerically single, in
possessing this unity which dates from the beginning, which does not
become what it was not, and which will never cease being what it is.
If indeed they cannot exist without some producing principle, that
principle will not derive them from matter. It will have to add to
them something from its own being. But if intelligible entities thus
have at times more, and at times less, perfection, they will change;
which would contradict their (nature, or) "being," which is to remain
identical. Why indeed should they become such as they are now, and why
should they not always have been such as they now are? Further, if
they be at times more or less perfect, if they "become," they are not
eternal. But it is granted that the Soul (as an intelligible being) is
eternal.