The functions proper to animals
Both plants and animals can be said to be self-movers: the plant causes itself to grow by taking nutrients from the environment; most animals enjoy local motion which it initiates in response to its environment. In the case of animals, the self-movement is more pronounced, being a change in place. But this kind of self-movement is itself the result of activities characteristic of animals. That is, the movements of animals is a way of responding to the environment that is more active than it is for plants since it requires an awareness of the environment and an inclination to move toward or away from what is perceived. Thus, knowledge and appetite are the two functions characteristic of animals.
The nature of the sensitive soul.
The proper operations of animals, namely, knowledge and appetite, are irreducible to those of vegetative life. Hence, animals are essentially different from and superior to plants. Since the operation follows the being, we can say that there exists a sensitive soul in animals which is the principle of these vital actions. This soul is the substantial form of the body. There exists only a single substantial form in each animal. Each animal is a single substance, a composite of matter (body) and form (soul). The actions of the animal belong to the whole animal, to the single substance composed of body and soul.
Because the sensitive soul depends upon the body for all its operations, and because operation follows being, we can conclude that the sensitive soul depends on the body for its being (existence). Consequently, as explained in natural philosophy, the sensitive soul comes into existence in dependence upon matter: i.e., it is educed from the potency of matter. Likewise, it ceases to be when the sensing organic body is corrupted. Properly speaking, it is the substance (the composite of body and soul) that is corrupted in death. We can say, however, that the soul ceases to be indirectly (per accidens) when the substance is corrupted.)
Sensation and the consequent operation of the sensitive soul are evidently accompanied with change in the body, Thus, in the act of vision, the pupil of the eye is affected by reflection of color, and similarly in the other senses. Hence it is clear that the sensitive soul has no per se operation of its own, and that every operation of the sensitive soul belongs to the composite. Wherefore they are not subsistent, for the operation of anything follows the mode of its being.(Summa Theologiae Ia, 75, 3.)