ose 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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SECOND ARTICLE [I, Q. 78, Art. 2]

Whether the Parts of the Vegetative Soul Are Fittingly Described As
the Nutritive, Augmentative, and Generative?

Objection 1: It would seem that the parts of the vegetative soul are
not fittingly described--namely, the nutritive, augmentative, and
generative. For these are called "natural" forces. But the powers of
the soul are above the natural forces. Therefore we should not class
the above forces as powers of the soul.

Obj. 2: Further, we should not assign a particular power of the
soul to that which is common to living and non-living things. But
generation is common to all things that can be generated and
corrupted, whether living or not living. Therefore the generative
force should not be classed as a power of the soul.

Obj. 3: Further, the soul is more powerful than the body. But the
body by the same force gives species and quantity; much more,
therefore, does the soul. Therefore the augmentative power of the
soul is not distinct from the generative power.

Obj. 4: Further, everything is preserved in being by that whereby it
exists. But the generative power is that whereby a living thing
exists. Therefore by the same power the living thing is preserved.
Now the nutritive force is directed to the preservation of the living
thing (De Anima ii, 4), being "a power which is capable o