Aquinas refers to that in virtue of which the patient is acted upon as passive potentiality; and he claims that to every type of active power, there corresponds a determinate type of passive power. This chapter considers Aquinas’s views on passive powers. The chapter first considers Aquinas’s views on the constituents of material substances that give rise to their passive potentialities for being acted upon. Aquinas holds that material substances have passive potentialities in virtue of both their matter and their qualitative forms. The chapter next considers Aquinas’s views on how a material substance’s passive potentialities are identified and distinguished from one another. Finally, the chapter argues that Aquinas thinks that a substance’s passive potentialities for undergoing action are the same as its potentialities for existing in determinate ways. For example, a pot of water’s potentiality for being heated is the same as its potentiality for being hot.

Aquinas thinks that natural efficient causes can act through the active powers of substances distinct from themselves. Aquinas identifies two different ways in which an efficient cause can operate through another cause’s power, namely as an instrumental cause or as a secondary cause. The chapter discusses Aquinas’s basic understanding of instrumental causality and secondary causality. Instrumental causes are employed by another cause, called a principal cause, to reach its end. In acting for a higher end, the instrument acts through the principal cause’s power. Secondary causes are like instruments insofar as they cannot act unless a higher cause exercises its power. However, secondary causes differ from instruments insofar as they act for their own ends. The chapter discusses Aquinas’s examples of instrumental and secondary causality in the natural world. Aquinas uses the notion of instrumental causality to understand how higher-level natural powers, such as the nutritive power, employ the actions of elemental powers, such as heat, to reach their ends. He regards terrestrial causes as secondary causes that act through the power of the heavenly bodies.

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.

Powers explain how agents are able to act. Yet, Aquinas thinks that we must posit something further in a natural agent to account for why it exercises its power whenever it is possible to do so. Natural inclination is that impetus within natural agents that determines them toward action. This chapter examines Aquinas’s understanding of natural inclination and the role it plays in efficient causation. The chapter first considers Aquinas’s views on what natural inclinations are and why they are necessary. The chapter next considers his views on how natural inclinations explain how natural agents act for the sake of ends. Even though natural agents cannot know the ends for which they act, Aquinas thinks that they nevertheless act for the sake of goals through their natural inclinations. Lastly, the chapter examines Aquinas’s views on the ultimate cause of natural inclinations. Aquinas maintains that natural inclinations, and the natures upon which they follow, must have their ultimate causal source in a being with cognition, namely God. The chapter analyzes Aquinas’s rationale for this view.

Aquinas thinks that not all instances of efficient causation are equivalent. Certain instances of efficient causation, namely per se cases, are the most fundamental and proper. Other instances of efficient causation happen in virtue of these cases. The chapter reconstructs Aquinas’s views on per se efficient causation. The chapter next examines Aquinas’s views on the temporal and modal relationship that obtains between per se natural efficient causes and their effects. The chapter shows that Aquinas thought that per se causes are simultaneous with their effects. Contemporary scholars debate whether Aquinas thought that natural efficient causes necessitate their effects. The chapter brings greater clarity to Aquinas’s views by examining his distinctions between different types of natural efficient causes and different types of necessity. Finally, the chapter considers Aquinas’s views on important relational conditions for efficient causation: the agent and the patient must be distinct and they must be in contact with each other. The chapter analyzes Aquinas’s arguments against self-motion and action at a distance.

This chapter is about Aquinas’s views on active powers. Aquinas uses the Latin term potentiae to refer to active powers in general. Accordingly, the chapter begins with Aquinas’s understanding of the distinction between potentiality and actuality and the different types of potentialities. The chapter next considers his views on how active potentialities are individuated. Aquinas claims that active potentialities are distinguished by the acts that immediately arise from them. The chapter then examines Aquinas’s views on what active powers are ontologically. Aquinas identifies active power with form. Forms are both that by which a substance is actual in a determinate way and that through which a substance is capable of causing the same type of form in another substance. Although Aquinas thinks that active powers are forms, he denies that every form is an active power for material change. For example, the form of redness is not an active power for making other substances red. The final sections of the chapter discuss Aquinas’s views about which forms are and are not active powers for initiating material change.

This study has covered a lot of details in Aquinas’s theories about efficient causation and causal powers. Thus, it might be useful to conclude by reiterating how his theories respond to some of the “big picture” philosophical questions about causation. One major question that arises when thinking about causation is: What is essential to the relationship between a cause and its effect? Put otherwise, what is it that ties certain phenomena together as cause and effect? What unifies disparate cases of causation as instances of causation? As noted in the introduction, many contemporary theories conceive of causation as a logical relationship between events. Attempts to specify this logical relationship are routinely defeated by counterexamples. Aquinas’s theory offers an alternative way of conceiving of the relationship between cause and effect. Instead of construing causation as a logical relationship, he sees causation as an ontological phenomenon. As we have seen, causes influence the being of their effects and effects depend on their causes for their existence. The unique way in which efficient causes influence the being of their effects is by action, namely an exercise of power. Aquinas recognizes, on the one hand, that there are primary and paradigm exercises of power, namely in per se instances of efficient causation, and yet, on the other hand, not all exercises of power bring about their effects in a uniform way. While per se causes are simultaneous with their effects, advising and preparing causes act prior to their effects. While unimpeded per se causes necessitate their effects, other efficient causes do not necessitate their effects. Despite their difference, what all instances of efficient causation have in common is that the cause influences the being of its effect by exercising a power in action. By conceiving of causation as an ontological relationship of dependence, Aquinas is able to find a common feature between pairs of causes and effects that bear different temporal, modal and logical relationships to each other. This gives his view flexibility to account for varied instances of causation which might pose counterexamples to more rigid views that define causation in terms of a single logical relationship.

This chapter examines Aquinas’s views on the ontological status of the actualization of an agent’s active power, namely its action, and the patient’s passive power, namely its passion. Aquinas claims that one and the same motion constitutes both an agent’s action and its patient’s passion. This chapter considers Aquinas’s motivations for defending the “action-passion sameness” thesis and his responses to common objections. The chapter also includes a solution to a longstanding interpretive difficulty regarding Aquinas’s views on the ontological status of action. Aquinas claims in some texts that actions are accidents in the agent as subject. This seems to conflict with his standard view that an agent’s action is the motion which it causes in its patient. While advancing a solution to this textual difficulty, the chapter proposes a novel interpretation of Aquinas’s understanding of the relationship between forms and accidents and the metaphysics of inherence.

Aquinas thinks that natural efficient causes can act through the active powers of substances distinct from themselves. Aquinas identifies two ways in which an efficient cause can operate through another cause’s power, namely as an instrumental cause or as a secondary cause. The chapter discusses Aquinas’s basic understanding of instrumental causality and secondary causality. Instrumental causes are employed by another cause, called a principal cause, to reach its end. In acting for a higher end, the instrument acts through the principal cause’s power. Secondary causes are like instruments insofar as they cannot act unless a higher cause exercises its power. However, secondary causes differ from instruments insofar as they act for their own ends. The chapter discusses Aquinas’s examples of instrumental and secondary causality in the natural world. Aquinas uses of the notion of instrumental causality to understand how higher-level natural powers, such as the nutritive power, employ the actions of elemental powers, such as heat, to reach their ends. He regards terrestrial causes as secondary causes that act through the power of the heavenly bodies.