Passions of the Soul
The powers of the soul are, roughly speaking, the human emotions. They are the motions educed in the sense appetites—the concupiscible and irascible appetites—by the objects perceived in the senses. Aquinas's main discussion of the powers of the soul is in ST I-II, qq. 22–48. There, we find out that there are, generally speaking, eleven passions of the soul—eleven basic ways in which the soul's appetites can be moved toward or away from external things.
Knowledge of the passions is crucial for moral philosophy, which seeks to bring the passions under the control of reason as far as is possible and to harness the force of the passions toward good ends. Strong passions can diminish or even wholly remove moral culpability, but they can also increase the merit of human action or exacerbate the blame due it (cf. ST I-II, q. 24). Without any passion whatsoever, according to Aquinas, no human outward activity is possible at all!