The idea that God is entirely simple and thus not composed in any way is central to St. Thomas's account of God. In Aquinas on Simplicity Peter Weigel investigates the metaphysics that underlie St. Thomas's account of divine simplicity in order to set divine simplicity within its correct theoretical context.
Weigel divides the book into six chapters, a conclusion and an appendix. The first chapter discusses Aquinas's accounts of divine simplicity in Summa contra gentiles, Compendium theologiae and Summa theologiae. Chapter two discusses the kinds of metaphysical composition which characterise created things but which divine simplicity excludes from God. The next three chapters examine the consequences of excluding metaphysical composition from God. Those consequences fall into two different kinds: what God's lack of composition excludes from God's being, and what God's lack of composition entails for the role God performs in Aquinas's metaphysics and theology. Accordingly, chapter three considers God as pure act, chapter four considers God as subsistent esse, and chapter five considers God as lacking matter or accidents. Chapter six examines how divine simplicity affects the way we can speak about God. The conclusion summarises the study and the appendix supplements the footnotes with some further material.
Several features of the book make it useful. First, Weigel offers an accessible introduction to many of the basic ideas of Aquinas's metaphysics. Hence, we find helpful discussions of matter and form, substance and accident, essence and existence, act and potency, pure act, subsistent esse and so on. Secondly, Weigel discusses some recent criticisms of Aquinas which are drawn from the analytical tradition in philosophy. That engagement is particularly fruitful in pp. 146–156 where Weigel discusses analytical objections to Aquinas's characterisation of God as subsistent esse and is able to show that those objections misinterpret Aquinas's view. Thirdly, Weigel discusses the role God plays in Aquinas's model of explanation in an interesting and helpful way. He contrasts Aquinas's model of explanation with Leibnitz's model and shows that Aquinas was primarily interested in accounting for the existence of things rather than providing a sufficient reason for every claim we can make about creation. Fourthly, Weigel shows that the importance of the concept of divine simplicity gradually increases throughout the course of Aquinas's work so that by the time of the Summa theologiae simplicity has become ‘the ontological precondition of the other major divine predicates’ (p. 37). Weigel does not pursue whether this increase in importance is a genuine departure from the views Aquinas held in Summa contra gentiles and Compendium theologiae or whether it is just a change of emphasis. This is regrettable but understandable for a book of this kind.
The main difficulty with the book is that in order to achieve its overall aim it needs to engage further with criticisms of Aquinas's view. Weigel, of course, is perfectly aware that Aquinas's account of divine simplicity faces several objections and he helpfully distinguishes those objections into three different types ‘one intrinsic and two systematic’ (p. 16). However, whilst Weigel's book ‘does not emphasise the systematic controversies that tend to be the focus of the current literature’ (p. 19) nevertheless, ‘an important suggestion of this work’ (ibid.) is that ‘a hard look at the doctrine itself is needed before much headway can be made in the systematic areas’ (ibid.). And presumably that ‘hard look’ is Weigel's examination of ‘the metaphysics of simplicity in Aquinas’ (ibid.) which involves ‘looking at some of the governing assumptions behind his (Aquinas's) entire philosophical approach to the divine nature’ (ibid.).
But if Weigel's point is that a general examination of Aquinas's account of divine simplicity will put the reader in a better position to assess the systematic objections raised against that account of simplicity, which is perfectly reasonable, then Weigel needs to show how the general examination of Aquinas's view undertaken actually does that. After all, why should anyone expend time and energy locating Aquinas's account of divine simplicity in its correct theoretical context if doing so offers no prospect whatsoever of being in a better position to assess any of the problems associated with that view? Indeed that Weigel aims the book at ‘the philosophically educated, non-specialist reader’ (p. 20) only reinforces the point. Such a reader might not be an Aquinas specialist but they could reasonably know something about the systematic objections raised against Aquinas's account of simplicity, and if the connection between Weigel's project and the objections to Aquinas's view is not explicit then such readers are likely to turn away.
Weigel has written an interesting book on an important topic. Whilst the book is unlikely to change the mind of anyone who disagrees with Aquinas's account it will be useful as an undergraduate textbook.
