passive power, passion, active powers, inclinations, transmutation
Aquinas used the term ‘agent’ referring to a created substance or to God. Aquinas's concept of substance explains that substances are set apart from accidents by the fact that substances are subsistent things. Aquinas believed that each substance belong to a particular species and has a complete nature common to any other members of that species that there may be. He also mentioned that there is a distinctive set of causal powers corresponding to each specific nature. Aquinas called the change (or motion) produced by the agent the ‘passion’. Aquinas considered active powers as real (though not necessarily physical) components of a thing that enable it to act in certain ways. A passive power is something posited to account for the fact that a thing is capable of being acted upon in a certain way, that is, to account for the fact that a thing is capable of undergoing a certain sort of passion. Aquinas claimed that every agent (living and nonliving) acts by intending some end. Aquinas did not think that inanimate objects do things out of an awareness of some goal. Aquinas distinguished two types of inclinations that include natural and voluntary. The types of natural inclinations are fire's inclination to heat and a stone's inclination to fall. A voluntary inclination is just any act of the will.
The Oxford Handbook of Aquinas