Western thinking about original sin gets its gravity from Augustine. Some orbit him. Others blast against him till they achieve escape velocity. Aquinas was in orbit, but in a distinctive path. I now explore Aquinas’s views. Aquinas believed in a historical Adam and Eve, and treated the Genesis account of the fall as literal, fly-on-the-wall, accurate history. To ease exposition, I speak within these assumptions.

Aquinas’s writings on normative ethics are vast, with 1,004 articles on virtue ethics and related matters in the Summa theologiae (ST) alone. These writings constitute an extraordinarily intricate picture of the kind of human life that Aquinas considers normative, but they also contain plenty of surprises, especially for those who assume that Aquinas is guided principally by the virtue ethics of Aristotle. Arguably the greatest of these surprises is that Aquinas’s writings on virtue ethics are not, in fact, simply about virtues. Instead, Aquinas’s virtues in the ST are integrated into a fourfold system of perfective attributes, namely virtues, gifts, beatitudes, and fruits (VGBF). In this chapter, I present a brief summary of this system and my interpretation of its meaning in the light of recent research.

Grace is a gratuitous divine gift that exceeds our human nature and allows us to obtain a supernatural, eternal good. Thomas Aquinas, who attempts to formulate the orthodox Christian teaching on grace, understands by it in one sense a stable disposition (habitus) infused by God into the soul that lifts human nature so as to partake in the divine nature. It is thus a created reality, not simply the fact of enjoying God’s favor. In a second sense, he understands by grace an aid (auxilium) of God moving us to know, will, or do something. While Aquinas’s terminology varies, scholars call the first kind ‘habitual grace’ and the second ‘actual grace.’1

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.

The aim of this chapter is to introduce Aquinas’s account of ultimate explanations in metaphysics. What are the constituent principles of created reality? What do they indicate about God in his unique existence, nature, and divine simplicity? How may one reasonably understand the Christian doctrine of the Trinity in light of the affirmation of divine simplicity? To consider these questions I will proceed in three stages, examining first Aquinas’s distinctive claims regarding the distinction of essence and existence in creatures; second his interpretation of the traditional Christian affirmation that the divine nature is simple; and third his concept of Trinitarian persons as subsistent relations, a teaching that helps illustrate the logical compatibility of Aquinas’s Trinitarian theology, his doctrine of God, and his metaphysics of creaturely composition.

Wherever Aquinas discusses mental life – cognition, perception, thought, knowledge, reasoning – his writing can seem like a trackless wilderness to the uninitiated. The texts overflow with technical Latin terms that come into English as impenetrable jargon: ‘intellect in actuality,’ ‘habit of science,’ ‘sensible species,’ ‘intelligible being,’ ‘intellected intention,’ ‘estimative power,’ ‘word of the heart.’ Even terms corresponding to familiar English terms, such as ‘belief,’ ‘judgment,’ ‘experience,’ ‘passion,’ ‘intention,’ or ‘perception,’ are used in confusing and unexpected ways.

The doctrine of the Incarnation is the Christian teaching that Jesus Christ, the man who was born of Mary and crucified under Pontius Pilate, was not merely a human, but was God incarnate – one person of the Holy Trinity. Retaining his divine nature, the Son of God took on, or assumed, in the technical language, a human nature, and thus became a real human, no less a human than you or I. Jesus Christ, then, on the traditional view that Aquinas inherited and defended, is one divine person with two complete natures. This chapter will focus on Aquinas’s metaphysical understanding of the Incarnation. For a discussion of the goal of the Incarnation – the regeneration of humans to right relationship with God – seein this volume, by Thomas Williams.1

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.

The nature of Aquinas’s ethical theory has often been the subject of debate among scholars. During much of the twentieth century, he was regarded as holding a natural law theory. In more recent years, recognition of his extensive discussion on virtue has led scholars to argue for a virtue-based account. Currently, an important debate centers on what counts as genuine virtues for Aquinas.