HKB
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.