Powers of the Soul

The powers of the soul are qualities (in the second species of quality; cf. ST I-II, q. 49, a. 2) in virtue of which a living thing is able to perform vital operations. If I put fire to a table or a stone, it does not feel anything, but if I put fire to my own hand, I feel heat! Thus, there must be something about my hand in virtue of which it feels whereas a stone or table does not. This something is called a power. 

Aquinas recognizes three vegetative powers belonging to all living things, at least twelve powers belonging to animals (sentient living things: [more than] five external senses; four internal senses; a locamotive power; and two appetitive powers), and three powers belonging only to humans. He divides these powers up by their respective objects—that is, the things toward which they are directed or by which they are activitated. This division, which is clearest in ST I, qq. 78–81, is represented in the diagram below.