mals more perfect than plants, and man than brute animals; and in
each of these genera there are various degrees. For this reason
Aristotle, _Metaph._ viii (Did. vii, 3), compares the species of things
to numbers, which differ in species by the addition or subtraction of
unity. And (De Anima ii, 3) he compares the various souls to the
species of figures, one of which contains another; as a pentagon
contains and exceeds a tetragon. Thus the intellectual soul contains
virtually whatever belongs to the sensitive soul of brute animals, and
to the nutritive souls of plants. Therefore, as a surface which is of
a pentagonal shape, is not tetragonal by one shape, and pentagonal by
another--since a tetragonal shape would be superfluous as contained
in the pentagonal--so neither is Socrates a man by one soul, and
animal by another; but by one and the same soul he is both animal and
man.

Reply Obj. 1: The sensitive soul is incorruptible, not by reason of
its being sensitive, but by reason of its being intellectual. When,
therefore, a soul is sensitive only, it is corruptible; but when with
sensibility it has also intellectuality, it is incorruptible. For
although sensibility does not give incorruptibility, yet it cannot
deprive intellectuality of its incorruptibility.

Reply Obj. 2: Not forms, but composites, are classified either
generically or specifically. Now man is corruptible like other
animals. And so the difference of corruptible and incorruptible which
is on the part of the forms does not involve a generic difference
between man and the other animals.

Reply Obj. 3: The embryo has first of all a soul which is merely
sensitive, and when this is removed, it is supplanted by a more
perfect soul, which is both sensitive and intellectual: as will be
shown further on (Q. 118, A. 2, ad 2).

Reply Obj. 4: We must not consider the diversity of natural things as
proceeding from the various logical notions or intentions, which flow
from our manner of understanding, because reason can apprehend one
and the same thing in various ways. Therefore since, as we have said,
the intellectual soul contains virtually what belongs to the
sensitive soul, and something more, reason can consider separately
what belongs to the power of the sensitive soul, as something
imperfect and material. And because it observes that this is
something common to man and to other animals, it forms thence the
notion of the genus; while that wherein the intell