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. _______________________
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 of preserving whatever receives it." Therefore we should not distinguish the nutritive power from the generative.
On the contrary, The Philosopher says (De Anima ii, 2,4) that the operations of this soul are "generation, the use of food," and (cf. De Anima iii, 9) "growth."
I answer that, The vegetative part has three powers. For the vegetative part, as we have said (A. 1), has for its object the body itself, living by the soul; for which body a triple operation of the soul is required. One is whereby it acquires existence, and to this is directed the generative power. Another is whereby the living body acquires its due quantity; to this is directed the augmentative power. Another is whereby the body of a living thing is preserved in its existence and in its due quantity; to this is directed the nutritive power.