Philosophical and theological motivations for Aquinas' work
At least two distinct purposes may be discerned in Aquinas' various writings on human action. One is to complete and correct Aristotle's treatment of it in the Nicomachean Ethics, to which he of course pays close and respectful attention. A second springs from his primary commitment to theology. Reflecting on what is said in the Scriptures and the writings of the Fathers about such topics as the fall of Adam, sin, conversion, and the operation of grace, theologians produced a body of doctrine about various aspects of human acts. To Aquinas' mind, this teaching settles certain questions authoritatively: as when it declares that voluntary human acts are commanded by their agents freely, and not by necessity. In addition it introduces certain concepts into the theory of action, for example, those of enjoyment and consent. Aquinas undertakes to incorporate these contributions of theology, where sound, into a revised Aristotelian theory.
Aristotelian causal theories
Aristotelian theories of action are causal, and causal in a distinctive way. To do something, to perform an act, is to cause something. And causing something is always to be investigated in terms of a pair of fundamental concepts, dynamis and energeia, which appear in Thomas' Latin as potentia (potency) and actus (act). The power or capacity of an object to cause something – whether a change of state, or a persistence in a state – largely determines what that object is. Brute animals are distinguished by their possession of powers of sensation and bodily movement.