This book is about thinking through philosophical and theological problems in the company of a great mind. It concerns one of philosophy’s greatest problems – Does God exist? This question raises others. Are there philosophical reasons for thinking that God exists? What kinds of reasoning, if any, might show that there is a God? What sort(s) of evidence “counts” for or against it? What is God? What do we mean by the term “God”? Much depends on what God is thought to be like. What, if anything, can be known about God’s nature? Is God’s existence or nonexistence something human reason can address? Is belief in God just a matter of faith? The sciences provide one highly regarded model of rationality. Can modern science speak to whether God exists?

One medieval thinker’s views on these questions profoundly shapes how other philosophers approach them through the centuries. Saint Thomas Aquinas (1225–1274) in his Summa theologiae offers five arguments for God’s existence. Known as the Five Ways, philosophers still invoke them in discussions about God. A vast amount has been written about them. Occasionally entire books, like this one, are devoted to these five tersely worded paragraphs.Footnote ¹ Why a whole book on the Ways? The Five Ways are notoriously difficult to understand. A careful reading requires familiarity with Aquinas’s body of thought. In addition, even scholars of Aquinas find parts of the Ways difficult to interpret. They often disagree with each other on what certain parts of the arguments are saying. Because it is hard to reach a detailed understanding of the arguments, their real merits are difficult to assess. The Ways often end up being criticized for positions Aquinas does not hold or not accomplishing aims he does not intend.

There are specific reasons the Five Ways are not easy to understand. Each argument is a single paragraph. Within each argument, there is often little explanation of or support for each premise. Premises are stated without warrant. The premises tend to be stated in brief, technical language, which can make it difficult to know what they are saying. The meaning of an important claim will sometimes depend on an ambiguous Latin word or phrase. Each argument is laden with positions and terms unfamiliar to many present-day readers, even to those with some philosophical training.

Turning to how the Five Ways are presented, the arguments receive no summary introduction. Aquinas does not discuss the particulars of the arguments immediately before or after he gives them. (He does frequently refer back to them in the Summa once he gives them.) Noting their terse style, some commentators see the Ways, or some of them, as shorthand sketches of longer arguments his original audience knew. Aquinas often does explain and defend the premises elsewhere in his writings. But inexperienced readers of Aquinas may not know this, and fewer know where to find these explanations.

Reading the arguments thus requires being familiar with a larger body of concepts and positions most people nowadays are not (even many trained philosophers). This background context spans Aquinas’s views in such areas of philosophy as logic, semantics, metaphysics, theory of knowledge, theory of causation, philosophy of nature, and other areas. At the heart of this supporting context is Aquinas’s metaphysics and his particular views on types of causes and how they account for their effects. If I misunderstand what the First Way means by the term “motion,” or the type of causal series Aquinas has in mind in the First Way, then there is little hope for me grasping the argument’s main points. There is even less hope of assessing the argument’s merits objectively.

An aim of this book is to recover for the contemporary ear this extensive background context needed to read the Five Ways. Only then can their real merits and possible weaknesses be appreciated. Another benefit to examining this background is to be able to think along with Aquinas about a range of important philosophical issues. Looking into questions with Aquinas has a way of exposing people to ideas and views that otherwise might have never crossed their mind.

In this work, I primarily focus on explaining how the Five Ways work. I look at Aquinas’s reasons for holding the premises in the arguments. I also look at objections to his positions and offer responses. However, there is often no room for a thorough airing of the positions. I will frequently represent different sides of an issue without much comment, leaving the reader to decide or pursue the matter independently. I often give reasons why one might see Aquinas’s views as persuasive. But I will also acknowledge how others for sophisticated reasons remain unconvinced of a position.

For the sake of brevity, I tend to focus more on the central terms and premises of each Way. I address the kinds of questions readers tend to raise. But, to keep this book to a reasonable length, some issues I will have little or no room to cover. My hope is that these discussions of the Ways at least help readers to think about some points or ideas they would not otherwise have noticed. Some may be inspired to read further in Aquinas. I hope all achieve some greater appreciation for the rich complexity and philosophical depth of the Five Ways, apart from whether one finds them persuasive.

In Chapter 1 I examine Aquinas’s views on the use of philosophy to reason about God’s existence. Aquinas contrasts this use of philosophy with believing things about God based on faith. Thus, I briefly go into his classic discussion of faith and reason in Summa theologiae Ia q.1. In Ia q.2 a.1–2, just before the Five Ways in Ia q.2 a.3, Aquinas looks at different ways of reasoning to God’s existence. Aquinas holds to a particular method of reasoning to God, which he contrasts with other approaches. Thus, he gives us a helpful preview of the type of demonstration he will use in the Five Ways.

Chapter 2 looks at Aquinas’s views on what types of causes there are and how causes produce their effects. A focus will be on Aristotle’s (384–322 BC) famous four causes. Another theme is what Aquinas considers an adequate causal account for why things exist and have the characteristics they do. These views set the stage, in the Ways, for God being the ultimate cause of things. Thus, questions about how far causal explanations must go, and the type of account called for, will figure prominently in this book. Chapter 3 digs deeper into these themes by looking at the great question in metaphysics, “Why is there something rather than nothing?” Why does anything exist? Given that so much already exists, why does it go on existing? These questions will be inroads to contrasting Aquinas’s views on accounting for the existence of things with the views of other influential thinkers.

Chapters 4–8 are devoted to treating each of the Five Ways in a separate chapter. In each chapter, I do a number of things. (1) First, I offer a translation of each Way from Aquinas’s Latin into English. (2) I then give my version of the argument’s premises. (3) Next, in an introductory section I offer a summary of the argument. I also anticipate the main issues the chapter will consider, usually issues with interpreting the premises and weighing common objections to them. (4) After the introductory section, I often present where Aquinas has used similar or parallel arguments to that Way, from earlier in his writings. (5) The bulk of each chapter sees different sections devoted to interpreting the main premises. (6) I also weigh some commonly made objections to the premises and offer how Aquinas might (or does) respond. Chapter 9 offers a retrospective on the themes in each chapter and concluding thoughts on reading the Five Ways.