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Objection 1: It would seem that there are not to be distinguished
five genera of powers in the soul---namely, vegetative, sensitive,
appetitive, locomotive, and intellectual. For the powers of the soul
are called its parts. But only three parts of the soul are commonly
assigned---namely, the vegetative soul, the sensitive soul, and
the rational soul. Therefore there are only three genera of powers in
the soul, and not five.
Objection 2: Further, the powers of the soul are the principles of
its vital operations. Now, in four ways is a thing said to live.
For the Philosopher says (De Anima ii, 2): "In several ways a
thing is said to live, and even if only one of these is present, the
thing is said to live; as intellect and sense, local movement and
rest, and lastly, movement of decrease and increase due to
nourishment." Therefore there are only four genera of powers of the
soul, as the appetitive is excluded.
Objection 3: Further, a special kind of soul ought not to be
assigned as regards what is common to all the powers. Now desire is
common to each power of the soul. For sight desires an appropriate
visible object; whence we read (Ecclus. 40:22): "The eye
desireth favor and beauty, but more than these green sown fields."
In the same way every other power desires its appropriate object.
Therefore the appetitive power should not be made a special genus of
the powers of the soul.
Objection 4: Further, the moving principle in animals is sense,
intellect or appetite, as the Philosopher says (De Anima iii,
10). Therefore the motive power should not be added to the above as
a special genus of soul.
On the contrary, The Philosopher says (De Anima ii, 3),
"The powers are the vegetative, the sensitive, the appetitive, the
locomotion, and the intellectual."
I answer that, There are five genera of powers of the soul, as above
numbered. Of these, three are called souls, and four are called
modes of living. The reason of this diversity lies in the various
souls being distinguished accordingly as the operation of the soul
transcends the operation of the corporeal nature in various ways; for
the whole corporeal nature is subject to the soul, and is related to it
as its matter and instrument. There exists, therefore, an operation
of the soul which so far exceeds the corporeal nature that it is not
even performed by any corporeal organ; and such is the operation of the
"rational soul." Below this, there is another operation of the
soul, which is indeed performed through a corporeal organ, but not
through a corporeal quality, and this is the operation of the
"sensitive soul"; for though hot and cold, wet and dry, and other
such corporeal qualities are required for the work of the senses, yet
they are not required in such a way that the operation of the senses
takes place by virtue of such qualities; but only for the proper
disposition of the organ. The lowest of the operations of the soul is
that which is performed by a corporeal organ, and by virtue of a
corporeal quality. Yet this transcends the operation of the corporeal
nature; because the movements of bodies are caused by an extrinsic
principle, while these operations are from an intrinsic principle; for
this is common to all the operations of the soul; since every animate
thing, in some way, moves itself. Such is the operation of the
"vegetative soul"; for digestion, and what follows, is caused
instrumentally by the action of heat, as the Philosopher says (De
Anima ii, 4).
Now the powers of the soul are distinguished generically by their
objects. For the higher a power is, the more universal is the object
to which it extends, as we have said above (Question 77, Article
3, ad 4). But the object of the soul's operation may be
considered in a triple order. For in the soul there is a power the
object of which is only the body that is united to that soul; the
powers of this genus are called "vegetative" for the vegetative power
acts only on the body to which the soul is united. There is another
genus in the powers of the soul, which genus regards a more universal
object---namely, every sensible body, not only the body to which
the soul is united. And there is yet another genus in the powers of
the soul, which genus regards a still more universal
object---namely, not only the sensible body, but all being in
universal. Wherefore it is evident that the latter two genera of the
soul's powers have an operation in regard not merely to that which is
united to them, but also to something extrinsic. Now, since whatever
operates must in some way be united to the object about which it
operates, it follows of necessity that this something extrinsic, which
is the object of the soul's operation, must be related to the soul in
a twofold manner. First, inasmuch as this something extrinsic has a
natural aptitude to be united to the soul, and to be by its likeness in
the soul. In this way there are two kinds of powers ---namely, the
"sensitive" in regard to the less common object---the sensible
body; and the "intellectual," in regard to the most common
object---universal being. Secondly, forasmuch as the soul itself
has an inclination and tendency to the something extrinsic. And in
this way there are again two kinds of powers in the soul: one---the
"appetitive"---in respect of which the soul is referred to
something extrinsic as to an end, which is first in the intention; the
other---the "locomotive" power---in respect of which the soul is
referred to something extrinsic as to the term of its operation and
movement; for every animal is moved for the purpose of realizing its
desires and intentions.
The modes of living are distinguished according to the degrees of
living things. There are some living things in which there exists only
vegetative power, as the plants. There are others in which with the
vegetative there exists also the sensitive, but not the locomotive
power; such as immovable animals, as shellfish. There are others
which besides this have locomotive powers, as perfect animals, which
require many things for their life, and consequently movement to seek
necessaries of life from a distance. And there are some living things
which with these have intellectual power---namely, men. But the
appetitive power does not constitute a degree of living things; because
wherever there is sense there is also appetite (De Anima ii, 3).
Thus the first two objectives are hereby solved.
Reply to Objection 3: The "natural appetite" is that inclination
which each thing has, of its own nature, for something; wherefore by
its natural appetite each power desires something suitable to itself.
But the "animal appetite" results from the form apprehended; this
sort of appetite requires a special power of the soul---mere
apprehension does not suffice. For a thing is desired as it exists in
its own nature, whereas in the apprehensive power it exists not
according to its own nature, but according to its likeness. Whence it
is clear that sight desires naturally a visible object for the purpose
of its act only---namely, for the purpose of seeing; but the animal
by the appetitive power desires the thing seen, not merely for the
purpose of seeing it, but also for other purposes. But if the soul
did not require things perceived by the senses, except on account of
the actions of the senses, that is, for the purpose of sensing them;
there would be no need for a special genus of appetitive powers, since
the natural appetite of the powers would suffice.
Reply to Objection 4: Although sense and appetite are principles of
movement in perfect animals, yet sense and appetite, as such, are not
sufficient to cause movement, unless another power be added to them;
for immovable animals have sense and appetite, and yet they have not
the power of motion. Now this motive power is not only in the appetite
and sense as commanding the movement, but also in the parts of the
body, to make them obey the appetite of the soul which moves them. Of
this we have a sign in the fact that when the members are deprived of
their natural disposition, they do not move in obedience to the
appetite.
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