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Objection 1: It would seem that besides the intellectual soul there
are in man other souls essentially different from one another, such as
the sensitive soul and the nutritive soul. For corruptible and
incorruptible are not of the same substance. But the intellectual soul
is incorruptible; whereas the other souls, as the sensitive and the
nutritive, are corruptible, as was shown above (Question 75,
Article 6). Therefore in man the essence of the intellectual soul,
the sensitive soul, and the nutritive soul, cannot be the same.
Objection 2: Further, if it be said that the sensitive soul in man
is incorruptible; on the contrary, "corruptible and incorruptible
differ generically," says the Philosopher, Metaph. x (Did. ix,
10). But the sensitive soul in the horse, the lion, and other
brute animals, is corruptible. If, therefore, in man it be
incorruptible, the sensitive soul in man and brute animals will not be
of the same "genus." Now an animal is so called from its having a
sensitive soul; and, therefore, "animal" will not be one genus
common to man and other animals, which is absurd.
Objection 3: Further, the Philosopher says, Metaph. viii
(Did. vii, 2), that the genus is taken from the matter, and
difference from the form. But "rational," which is the difference
constituting man, is taken from the intellectual soul; while he is
called "animal" by reason of his having a body animated by a sensitive
soul. Therefore the intellectual soul may be compared to the body
animated by a sensitive soul, as form to matter. Therefore in man the
intellectual soul is not essentially the same as the sensitive soul,
but presupposes it as a material subject.
On the contrary, It is said in the book De Ecclesiasticis
Dogmatibus xv: "Nor do we say that there are two souls in one man,
as James and other Syrians write; one, animal, by which the body is
animated, and which is mingled with the blood; the other, spiritual,
which obeys the reason; but we say that it is one and the same soul in
man, that both gives life to the body by being united to it, and
orders itself by its own reasoning."
I answer that, Plato held that there were several souls in one body,
distinct even as to organs, to which souls he referred the different
vital actions, saying that the nutritive power is in the liver, the
concupiscible in the heart, and the power of knowledge in the brain.
Which opinion is rejected by Aristotle (De Anima ii, 2), with
regard to those parts of the soul which use corporeal organs; for this
reason, that in those animals which continue to live when they have
been divided in each part are observed the operations of the soul, as
sense and appetite. Now this would not be the case if the various
principles of the soul's operations were essentially different, and
distributed in the various parts of the body. But with regard to the
intellectual part, he seems to leave it in doubt whether it be "only
logically" distinct from the other parts of the soul, "or also
locally."
The opinion of Plato might be maintained if, as he held, the soul
was supposed to be united to the body, not as its form, but as its
motor. For it involves nothing unreasonable that the same movable
thing be moved by several motors; and still less if it be moved
according to its various parts. If we suppose, however, that the
soul is united to the body as its form, it is quite impossible for
several essentially different souls to be in one body. This can be
made clear by three different reasons.
In the first place, an animal would not be absolutely one, in which
there were several souls. For nothing is absolutely one except by one
form, by which a thing has existence: because a thing has from the
same source both existence and unity; and therefore things which are
denominated by various forms are not absolutely one; as, for
instance, "a white man." If, therefore, man were 'living' by
one form, the vegetative soul, and 'animal' by another form, the
sensitive soul, and "man" by another form, the intellectual soul,
it would follow that man is not absolutely one. Thus Aristotle
argues, Metaph. viii (Did. vii, 6), against Plato, that if
the idea of an animal is distinct from the idea of a biped, then a
biped animal is not absolutely one. For this reason, against those
who hold that there are several souls in the body, he asks (De Anima
i, 5), "what contains them?"---that is, what makes them one?
It cannot be said that they are united by the one body; because rather
does the soul contain the body and make it one, than the reverse.
Secondly, this is proved to be impossible by the manner in which one
thing is predicated of another. Those things which are derived from
various forms are predicated of one another, either accidentally, (if
the forms are not ordered to one another, as when we say that something
white is sweet), or essentially, in the second manner of essential
predication, (if the forms are ordered one to another, the subject
belonging to the definition of the predicate; as a surface is
presupposed to color; so that if we say that a body with a surface is
colored, we have the second manner of essential predication.)
Therefore, if we have one form by which a thing is an animal, and
another form by which it is a man, it follows either that one of these
two things could not be predicated of the other, except accidentally,
supposing these two forms not to be ordered to one another---or that
one would be predicated of the other according to the second manner of
essential predication, if one soul be presupposed to the other. But
both of these consequences are clearly false: because "animal" is
predicated of man essentially and not accidentally; and man is not part
of the definition of an animal, but the other way about. Therefore of
necessity by the same form a thing is animal and man; otherwise man
would not really be the thing which is an animal, so that animal can be
essentially predicated of man.
Thirdly, this is shown to be impossible by the fact that when one
operation of the soul is intense it impedes another, which could never
be the case unless the principle of action were essentially one.
We must therefore conclude that in man the sensitive soul, the
intellectual soul, and the nutritive soul are numerically one soul.
This can easily be explained, if we consider the differences of
species and forms. For we observe that the species and forms of things
differ from one another, as the perfect and imperfect; as in the order
of things, the animate are more perfect than the inanimate, and
animals 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 to Objection 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 to Objection 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 to Objection 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 (Question 118, Article 2, ad 2).
Reply to Objection 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 intellectual soul exceeds the sensitive soul, it
takes as formal and perfecting; thence it gathers the "difference" of
man.
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