Whether There Are to Be Distinguished Five Genera of Powers in the
Soul?
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.
Obj. 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.
Obj. 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.
Obj. 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 (Q. 77, A. 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 objections are hereby solved.
Reply Obj. 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 Obj. 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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