Aquinas's Way to God: The Proof in "De ente et essentia" by Gaven Kerr

Gaven Kerr's Aquinas's Way to God is a book-length exposition and defense of the compact argument for God's existence that Aquinas presents in De ente et essentia. Such a book is much needed. Aquinas's argument is both a powerful theistic proof and crucial to a proper understanding of his conception of God, but it is little-known outside Thomistic circles. It also raises a number of tangled metaphysical and interpretive questions. A work that carefully unpacks Aquinas's reasoning, and interacts along the way with both the Thomistic literature and relevant literature in contemporary analytic philosophy, is long overdue. Kerr's book provides exactly this. It is excellent and pleasing, and can be read with profit both by those already familiar with Aquinas and those coming to him for the first time.

What follows are summaries of each of Kerr's chapters, and then a few points of criticism. Chapter 1 is devoted to an analysis of the argumentation for the real distinction between essence and esse (a thing's being or existence) in chapter 4 of De ente. Commentators typically identify two stages of this argumentation. The first, sometimes called the intellectus essentiae argument, appeals to the circumstance that we can know a thing's essence without knowing whether or not it exists. The second stage argues that a thing whose essence is identical to its esse would necessarily be unique, so that for any type of thing of which there is or could be more than one instance, its essence and esse must be distinct.

Kerr rejects the view of Joseph Owens that Aquinas's argumentation for the real distinction presupposes God's existence, which would entail that it cannot be used as the basis for a proof of God's existence. Another point of disagreement among Thomists is whether the first or intellectus essentiae stage of argumentation constitutes a separate, stand-alone argument for the real distinction, or instead merely establishes a conceptual distinction as a preamble to the second stage's establishment of the real distinction. Kerr endorses the latter view. He also responds to a potential Aristotelian objection to the Thomistic argument for the real distinction, posed by David Twetten.

In chapter 2, Kerr provides an exposition of the Thomistic understanding of essence, showing how it builds on but modifies Aristotle's conception of form and matter. Whereas previous Aristotelians tended to identify essence with form, Aquinas takes the matter of a material object also to be part of its essence. Kerr also discusses how Thomistic essentialism differs from, and is superior to, the versions of essentialism debated in contemporary analytic philosophy (with their excessive reliance on the notion of possible worlds and tendency toward Platonism).

Chapter 3 gives an account of the Thomistic notion of esse. Kerr explains how, by making esse rather than form the fundamental principle of actuality, Aquinas goes beyond Aristotle, and that by taking a thing's existence to amount to participation in esse he takes on board a key Platonic notion. However, since participation is interpreted by Aquinas in causal terms and tied to the Aristotelian idea of the limitation of act by potency, the resulting view is neither Aristotelian full stop nor Platonic full stop, but distinctively Thomistic. Kerr also compares and contrasts Aquinas's understanding of esse with the accounts of existence associated with Meinong; with Frege, Russell, and Quine; and with more recent analytic philosophers like David Lewis and Nathan Salmon. Kerr argues that each of these alternatives is deficient in various ways and that Aquinas's position is not only still defensible, but superior to them.

Aquinas's account of efficient causality is the subject of chapter 4. Kerr attributes to Aquinas a "causal principle" to the effect that a thing's properties are to be explained either as deriving from...