This chapter sets the stage by offering a general account of Aquinas’s understanding of scientia as a good habit of mind that has as its proper act the act of demonstrating. While the emphasis this chapter places on the psychological status of science and demonstration is somewhat novel vis-à-vis the secondary literature, the core claims it makes concerning scientia, quia demonstration, and propter quid demonstration are not.

This chapter raises and answers the question of what Aquinas thinks the subject of metaphysics is. Formally, he thinks it is whatever can be known through natural, strictly immaterial principles. Materially, he thinks it is common being – that is, the analogical community of actual substance with what is accidental and what is potential. Adequately (for neither the formal nor the material account is adequate on its own – something that has not been sufficiently appreciated in the secondary literature), Aquinas thinks the subject of metaphysics is common being insofar as it can be known through natural, strictly immaterial principles.

The Introduction raises the question of what the task, job, or purpose of metaphysics is and, by extension, what the metaphysician should aspire to do. It then canvasses different conceptions of metaphysics that answer these questions in different ways, noting how some answers are bolder and more ambitious than others. Among the boldest and most ambitious of these conceptions of metaphysics is that of Thomas Aquinas, who held both that metaphysics studies everything and that it can do so in such a way as to produce rigorous knowledge of the highest sort – what Aristotle would call unqualified episteme.

This chapter overturns the longstanding consensus that Aquinas identified the transcendentals of De veritate q. 1, art. 1 – that is, thing, one, something, true, and good – as the properties of being. Instead, they should be identified with disjunctive pairs like necessary or contingent, universal or particular, prior or posterior, one or many, and actual or potential. This radical revision in how we think of Aquinas’s account of the properties of being brings with it a corresponding reorientation of Thomistic metaphysics – for if what is primarily “at issue” in Aquinas’s metaphysics is the demonstrability of these disjunctive properties, then Thomistic metaphysics will place a far higher premium on themes like the diversity of being and ontological degree than is typically presented.

This chapter analyzes Aquinas’s account of the principles of metaphysics – both the causal principles of common being and the scientific principles (i.e., the per se nota propositions) from which metaphysical reasoning proceeds. Because the subject-matter of Aquinas’s metaphysics has real efficient, formal, and final causes, it is possible to formulate corresponding real causal definitions of that subject, which can, in turn, serve as terms used to construct the primitive principles of metaphysical science. This means is that it is possible for Aquinas’s metaphysics to be “scientific” in the strongest sense in which he understood the word – that is, that Aquinas’s metaphysics can meet even the most rigorous standards of Aristotelian episteme.

This chapter examines Aquinas’s account of will in God, showing how it builds on his teaching about knowledge. For Aquinas, will follows from intellect: as truth is the object of understanding, so good is the object of desire. Since God knows himself as perfect goodness, he necessarily delights in himself and thus has will essentially. God also wills creatures, yet not by necessity: creation is free, not compelled by divine nature or by anything external. Aquinas contrasts divine willing with human volition, stressing that while human will involves deliberation and change, God’s will is identical with his essence. Love, as the first movement of will, is also properly ascribed to God: God loves creatures insofar as he wills their existence and goodness, though unequally. The chapter also considers modern critiques – such as Kretzmann’s appeal to Dionysius and Swinburne’s theistic personalism – arguing that Aquinas avoids anthropomorphism by rooting will in divine simplicity.

This chapter focusses on Aquinas’s philosophical arguments for the existence of God. It begins by noting where he offers such arguments. Then it notes his general strategy in presenting them. Unlike many philosophers, Aquinas argues that the claim that we can only know that God exists by means of demonstrative arguments from certain facts about the universe to the conclusion that God must exist as cause of these facts. This chapter explains what that view amounts to for Aquinas and compares and contrasts it with other philosophical approaches to the question ‘How can we know that God exists’. The chapter then turns in detail to Aquinas’s arguments for God’s existence in his De Ente et Essentia (4) and his Summa Theologiae, 1a, 2, 3. The chapter explains how these arguments run and comments on what can be said for and against them.

This chapter explores Aquinas’s account of God’s perfection, goodness, and the problem of evil. Aquinas argues that God is perfect because he lacks all potentiality and is wholly actual, with every creaturely perfection pre-existing eminently in him. God is good, not as a member of a genus, but as the Creator whose nature grounds and causes the goodness of creatures. Goodness, Aquinas maintains, is convertible with being, while evil is privation of good – either as suffering (malum poenae) or wrongdoing (malum culpae). He denies that God directly causes evil, holding instead that evils suffered arise incidentally from the flourishing of other beings, while evils done derive from human free will. The chapter contrasts Aquinas’s approach with modern debates – such as Mackie’s logical argument from evil, Rowe’s evidential argument, and Plantinga’s free will defence—showing how Aquinas resists theodicy by emphasising God’s transcendence and the derivative character of creaturely goodness and freedom.

This chapter examines Aquinas’s doctrine of divine simplicity, situating it within the broader landscape of classical theism in contrast to theistic personalism. After outlining Aquinas’s conviction that while God’s existence can be demonstrated, God’s essence remains incomprehensible, the discussion turns to Summa Theologiae, 1a, 3, where Aquinas denies that God is composite in any respect. He argues that God is not a body, nor composed of form and matter, essence and existence, substance and accidents, or genus and difference. Instead, God is pure actuality, identical with his essence and existence, and thus utterly simple. The chapter compares these claims with parallel treatments in the Sentences, Summa Contra Gentiles, and Compendium Theologiae. Finally, it surveys modern objections – particularly from Alvin Plantinga, Richard Swinburne, and others – that divine simplicity undermines God’s personhood, freedom, or responsiveness. The chapter concludes that many such criticisms misrepresent Aquinas, who presents simplicity chiefly as a way of ruling out what God cannot be.

This chapter explores Aquinas’s account of knowledge, first in creatures and then in God. Aquinas distinguishes between scientia (understanding) and cognitio (awareness), situating knowledge within the broader framework of living beings, sensation, and intellect. For him, knowledge entails the immaterial reception of form: animals possess cognition through sensation, while humans attain scientia by grasping natures universally and immaterially. Aquinas links knowledge with immateriality, arguing that intellect expands in proportion to freedom from matter. Turning to God, Aquinas contends that divine knowledge is identical with God’s essence, wholly immaterial, and thus unlimited. Unlike human discursive reasoning, God’s knowledge is simple, immediate, and comprehensive: God knows himself perfectly, and in knowing himself as Creator, knows all things, including non-existent and future contingent beings. The chapter concludes by examining Aquinas’s treatment of truth, omniscience, and divine causality, showing how God’s knowledge differs fundamentally from creaturely.

This chapter provides an account of Aquinas’s life and works. It then explains his intellectual perspectives as an author while noting how he distinguishes between faith and reason. It notes what he means by ‘divine revelation’ and why he thinks that to be important. It also explains what he has to say about ‘sacra doctrina, ‘articles of faith’, and ‘preambles of faith. The chapter then explains why Aquinas can be thought to be an advocate of ‘natural theology’ and why he takes it to be important. Finally, it provides an introduction to some key terms which recur in his philosophy of God, including ‘ens’, ‘ens per se’, ‘ens per accidens ‘form’, ‘substantial form’, ‘accidental form’, ‘matter’, ‘substance’, ‘accident’, ‘cause’, ‘efficient cause’, ‘final cause’, ‘formal cause’, ‘material cause’, ‘effect’, ‘essence’, ‘nature’, ‘substantial change’, ‘accidental change’, ‘sensation’, ‘individuation’, and ‘being’ (esse).

This chapter examines Aquinas’s account of God as omnipotent and eternal. Aquinas distinguishes active from passive power, attributing only active power to God, who as pure actuality is wholly the source of effects. Divine omnipotence, he argues, does not mean God can do just anything, but that God can bring about all that is absolutely possible – that is, all that does not involve contradiction. Hence God cannot undo the past or contradict his own immutability, though he can work miracles, which Aquinas understands not as interventions but as events beyond the natural capacities of creatures. The chapter considers modern critiques, such as Geach’s challenges to defining omnipotence and Plantinga’s free-will defence, and contrasts them with Aquinas’s view that divine causality grounds rather than negates human freedom. Turning to eternity, the chapter explains Aquinas’s claim that God is utterly unchangeable and ‘altogether at once’ (tota simul) thereby transcending time without negating his causal presence in history.

This chapter investigates Aquinas’s account of how human beings can meaningfully speak about God despite the limits he takes human understanding to have. Focusing on Summa Theologiae, 1a, 3, the chapter explains how Aquinas argues that while our language originates in creaturely experience, it can nonetheless be applied to God in ways that are not merely metaphorical. The chapter also notes how Aquinas distinguishes between nominal and real definitions, between what a word signifies (res significata) and its mode of signification (modus significandi), and defends the view that terms such as ‘good’ or ‘wise’ can be predicated of God properly, though always imperfectly. Central to the chapter is Aquinas’s thinking on analogy, which mediates between univocal and purely equivocal predication, allowing words applied to creatures and God to enable us to speak truly about God without rejecting belief in divine simplicity. The chapter also explores later philosophical debates while highlighting how Aquinas’s nuanced account of analogy continues to illuminate the possibility of meaningful God-talk.

This chapter explores Aquinas’s account of God as Creator, both philosophically and theologically. Aquinas argues that everything other than God depends on divine causality not only for its origin but for its continued existence. Central here is the distinction between essence and existence: creatures have esse only by receiving it directly from God, whereas God is ipsum esse subsistens. Creation, therefore, is not the modification of pre-existing matter but the bringing into being from nothing (ex nihilo). The chapter examines how Aquinas differentiates divine causality from creaturely causality, allowing for genuine secondary causes while insisting that only God creates. It also considers Aquinas’s nuanced position on the eternity of the world, maintaining that reason cannot prove whether creation had a temporal beginning, though revelation affirms it. Finally, the chapter engages modern objections – such as existential inertia and the coherence of ‘creation from nothing’ – and shows how Aquinas’s metaphysics continues to challenge such critiques.

This chapter addresses the question of how to make sense of original sin in a manner that takes seriously the challenge of evolutionary biology. Can we retain a notion of original sin without having to accept the beliefs in monogenesis and a historical Fall from a state of paradisical innocence? In a first historical-theological section the views of Thomas and his predecessors are outlined. A second, more constructive-theological section proposes a notion of original sin without a historical Fall. Following McFarland, it is argued that we cannot separate an imperfect human nature from personhood. Drawing on early texts from Thomas, the rest of the chapter considers objections against the proposal that we have been created, not in a perfect manner, but as imperfect beings in need of grace. Can we still affirm the goodness of God if we have been created as less than perfect? Does this proposal suggest that God is somehow obligated to offer us the remedy of grace?

This chapter draws out the sacrificial implications of Aquinas’s account of love. By combining Aquinas’s theory with ideas from Philip Pettit about the robustness of love’s behavioral implications across possible worlds, the chapter presents a metaphysical framework for understanding why human love is essentially sacrificial. It is argued that the sacrificial dimension of human love is not contingent on the existence of sin and evil.