Q. 75, art. 3: Whether the Souls of Brutes are Subsistent
I. Brute souls are subsistent
(Argument by analogy)
A. The souls of brutes and humans are identical as regards subsistence
(Modus Ponens)
i.
If man and brute animals are identical in genus, then their souls are identical as
regards subsistence.1
ii. Man and brute animals agree in genus
B. The human soul is subsistent.
II. Brute souls are subsistent
(Barbara)
A. Whatever apprehends sensibles without a body is subsistent.
B. Souls of brutes apprehend sensibles without a body
(Barbara)
i.
Sensitive soul apprehends sensibles without a body.
(Argument by analogy)
a. Intellectual soul apprehends intelligibles without a body
b. The intellectual soul is to intelligibles as sensitive soul is to sensibles.2
ii. Souls of brutes are sensitive
III. Brute souls are subsistent.
(Barbara)
A. Whatever has an immaterial operation is subsistent
B. The soul of a brute has an immaterial operation
(Barbara)
i.
Every immaterial being has an immaterial operation.
ii. Every soul of a brute is immaterial
(obversion)
a. No soul of a brute is a body
(Celantes)
I) No body moves a body3
1
Aquinas denies this premise. Whether or not a being is subsistent or not depends on their form. But the form is
taken from the species, not the genus. Therefore, that a being belongs to a given genus does not determine
whether it's form is subsistent or not. Since human beings and other animals belong to different species, they
need not be the same as regards the subsistence of their souls.
2
Aquinas takes this premise to be ambiguous, since it does not name the relevant similarity between sensitive and
intellective souls. Aquinas states that sense and understanding are relevantly similar in this: that they are both in
potency towards their respective objects. But that a faculty perform its operation without a body does not follow
from its being in potency towards an object, and so the analogy does not support the conclusion.
Furthermore, if the sensitive soul performed its operation without the aid of a body, then it would not be affected by
bodily changes. But the sensitive soul is affected by bodily changes: the eye cannot stare at the sun without
causing damage to itself; adding too many spices to one's food deadens one's taste buds. By contrast, the intellect
is not weakened, but strengthened, by attending to objects that are highly intelligible. For example, learning to
solve a very difficult math problem, even if it may be fatiguing for the moment, strengthens one's intellectual
prowess in the long run.
3
Aquinas denies this premise. The minor premise with which this premise is paired suggests that the objector is
thinking of cases like locomotion—cases where the soul moves the body as efficient cause. So in order to deny
the premise, Aquinas must show that this motion is dependent on a body.
He accomplishes this in two steps: first, he reduces efficient causality to final causality; second, he shows that the
activity of the sensitive soul, as regards its final cause, requires a body.
In order to prove the first, he divides motive force into two kinds: appetitive and executing. locomotion is a kind of
executing motion. But every executing motion is itself a response, and therefore consequent on a being moved.
Therefore, Aquinas thinks, following Aristotle, that motion of this kind is not self-explanatory.
And so it is necessary to ask: what gives rise to this kind of motion in the sensitive soul? Aquinas' answer is that a
sensitive soul is moved by its appetitions, which are nothing other than its essential operations, i.e. sensations. So
having reduced the executing motion of sensitive souls to sensation, he next moves show that sensation depends
on a body.
II) The soul of a brute moves a body
Summary
Aquinas answers the question of whether the brute soul is subsistent in two steps: first, he
reports the opinions of the ancient authorities on the question; second, he puts forth an argument
in favor of the position of Aristotle.
Aquinas divides the ancient authorities in accordance with their answers to two questions:
first, whether sensation is distinct from intellection or understanding; second, presuming that
they are distinct, whether sensation is a per se operation of the sensitive soul.
To the first question, Aquinas states that a group of philosophers he calls the ancient
philosophers—that is, the group of philosophers that we today refer to as the pre-socratics—
uniformly held that there was no distinction between sensation and intellection; Plato and
Aristotle, on the other hand, both distinguished between sensation and intellection. To the second
question, Plato affirmed that sensation is the per se operation of the sensitive soul, while
Aristotle denied it.
The relevance of the second question will be made clear in the explication of Aquinas’
proof. But the relevance of the first question to the question of whether the souls of brutes are
subsistent is not so immediately clear. But it seems to be as follows. Any person who failed to
distinguish between intellection and sensation would, on account of this, have to give the same
answer to the question of whether sensation and intellection is a per se operation of the soul.
Now, it is more immediately apparent that sensation is not a per se operation of the soul (i.e. that
it takes place via corporeal organs) than that intellection is (i.e. that it does not make use of a
corporeal organ for its essential operation). Therefore, the person who failed to distinguish
between sense and understanding would be more likely than not to deny intellectual beings a per
se operation than to suggest that sensate beings have such an operation. According to Aquinas,
this seems to be just what the pre-Socratics did.
The previous question has already given us an idea of what a being must be in order to be
subsistent: it must have an operation which is uniquely its own. So, to show that the sensitive
soul is not subsistent, Aquinas must show that the sensitive soul does not have an operation
which is uniquely its own.
Aquinas does this by proposing the following argument. Sensitive operations are always
accompanied by bodily change. But any operation always accompanied by bodily change is a
bodily operation. Therefore, sensations are bodily operations, that is, operations performed by or
through a body. But no operation performed by or through a body is an operation performed only
by the soul—that is, no such operation is a per se operation of the soul. Therefore, sensations are
not per se operations of the soul. Safely presuming that operations lower than sensation (e.g. the
taking in of nutrients) are also not per se operations, we can thus conclude that no (merely)
sensitive soul has a per se operation, that is, that every sensitive soul lacks a per se operation.
And nothing lacking a per se operation is subsistent. Therefore, no merely sensitive soul is
subsistent. But the souls of brutes are merely sensitive. Therefore, no soul of a brute is subsistent.
This concludes Aquinas' argument.
Aquinas takes the truth that sensation involves bodily movement to be relatively uncontroversial, so he does not go
to great lengths to support it. He merely takes anger and joy as examples of movements in the sensitive soul that
cannot take place without bodily changes. Since these seem to be the highest operations of the sensitive soul,
and these are require a body, it follows a fortiori that every operation of the sensitive soul requires a bodily cause.