The introduction explains the significance of the study’s topics, efficient causation and causal powers. It explains why a study on Aquinas’s views of these topics is needed. It also describes the methodology for reconstructing Aquinas’s views and the organization of the book.

This chapter provides an introduction to Aquinas’s views on efficient causation and causal powers, as well as some background and context necessary for appreciating his views. The chapter first introduces Aquinas’s views on the nature of the relationship between an efficient cause and its effect and the various elements involved in paradigm cases of efficient causation. After presenting an overview of Aquinas’s theories, the chapter next contrasts Aquinas’s views with competing historical theories of causation. Comparison with these other theories helps to highlight what is philosophically significant in Aquinas’s theories. The chapter also discusses Aquinas’s sources and situates his views relative to medieval debates about causation. This background provides some context for appreciating what is original or controversial in Aquinas’s theories. Finally, the chapter includes an introduction to the technical terminology that Aquinas uses to express his views on efficient causation and causal powers.

This chapter discusses the relation between causation and moral responsibility. We generally hold adult human beings morally responsible for their actions, yet those action are also events in the natural world, enmeshed in causal chains that extend backwards in time long before the agent’s birth. If the causes in those chains necessitate their effects, it would appear that we must either give up the view that humans are morally responsible for their actions, or embrace the paradoxical view that humans are morally responsible for actions necessitated by events over which they have no control. Tuozzo argues that Aristotle’s causal theory avoids this dilemma by recognising two distinct types of causal chain or nexus. In one of these, the links between cause and effect are indeed necessary, from beginning to end. But chains of this sort are necessarily finite, with a definite beginning and end. Each of these necessary, finite causal chains are also enmeshed in a different sort of causal nexus, one that does extend indefinitely into the past. But this sort of indefinite causal chain is possible only because it contains links that are not necessitated. This enables Aristotle to account for moral responsibility by locating the necessitating cause of a human action in the agent herself. Nonetheless, Tuozzo concludes, Aristotle’s theory does have the paradoxical implication that, although the state of the world at a given time does not necessitate all subsequent events, a complete description of it would, in principle, allow all subsequent events – including human actions – to be predicted.

This chapter analyzes the key notion of the “human act” (actus humanus) around which Aquinas’s action theory revolves. It argues that, for Aquinas, the general term ‘act’ is used broadly to denote any power-exercise in nature, whether in the animate or the inanimate domain. Given this broad scope of the term ‘act,’ the chapter considers, in a next step, what sets human acts apart from other power-exercises. It argues that, according to Aquinas, one differentiating feature is that a human act is a hylomophically organized composite of two power-exercises, where a volition that he refers to as “use” is the formal component and a power-exercise caused by volition that he refers to as the “commanded act” is the material component. However, as the chapter also shows, Aquinas chiefly relies on an aetiological criterion to differentiate human acts from other power-exercises, arguing that a human act is explained by a preceding act of free choice. It then shows that choice likewise displays a hylomorphic structure, for Aquinas. It is a volition materially speaking, and its form is the preferential character that this volition inherits from a preceding judgment that Aquinas refers to as the “judgment of choice.”