The most important question in sacramental theology is undoubtedly that of the relationship between the sacraments and the grace which accompanies them. Broadly speaking two sorts of answers have been proposed: either the sacraments have some causal role in the bestowal of grace or they do not. In the latter case sacraments tend to be described as occasions for the bestowal of grace, or as conditions of varying strength that accompany such a bestowal, or as elements of a divine pact which leads to grace, or as some combination of these (p. 28). In the former case an account of their causal efficacy will be required which in the Thomist tradition at least has always been provided through instrumental causality and about which this book is concerned.

The book is divided into four chapters. The first surveys the main approaches to sacramental efficacy and is largely introductory. It examines the influence of St. Augustine pointing out that subsequent approaches tended to be proposed as interpretations of his work. It highlights the distinctive Franciscan and Thomist approaches, characterising the former as less concerned with the sacraments’ causal efficacy, the latter as more so. It points out that Trent only excluded occasionalist approaches and thus left a wide range of options for catholic theology to pursue. It also charts the post-World War II rise of the more experiential sacramental theologies of Cassel and Schillebeeckx and the corresponding, albeit mistaken, decline of interest in the question of sacramental efficacy.

The second and third chapters are the heart of the book, though. Together they offer a detailed examination of Aquinas's theory of sacramental instrumental causality beginning with the Sentence-Commentary in chapter two and continuing through the De Veritate, the De Potentia Dei, and the Summa Theologiae in chapter three. Chapter three also introduces the main commentatorial precisions and developments of Aquinas's position. Whilst chapter four considers moral causality as a key response to the Thomist view, as well as outlining the commentatorial response to moral causality.

The book is a delightful piece of work. It will benefit anyone with a general interest in the sacraments and be particularly attractive to the reader seeking to discover the sacramental theology of the Thomist tradition. Lynch does a good job of highlighting the importance of sacramental efficacy, insisting that unless the ‘core metaphysical questions which concern the sacraments’ (p. 200) are addressed, alternative approaches – whatever their merits may be – simply will not improve on the supposed faults of those that do. There is also a fascinating discussion of the contrast between organum and instrumentum in the Sentence-Commentary and the Summa Theologiae (pp. 105-110) and its application to the sacraments. And by bringing Aquinas's commentators into the scope of the work – not just limiting it to the Common Doctor – Lynch reminds us of the vitality of Thomism in the history of the Church and the intrinsic value of the commentatorial tradition.

There are areas where a reader is likely to want to press Lynch further though. One such area concerns Lynch's analysis of St. Thomas’ Sentence-Commentary argument that sacramental instrumental causes can only be dispositive (pp. 82-83). It is certainly true that no instrumental causality is involved in creation, at least when it is understood as the emanation of the whole of being from a universal cause. It is also true that St. Thomas thought human parents only have a dispositive causal role in the production of their offspring. But it is not clear how we can infer that sacramental instrumental causes can only be dispositive from either claim.

A reasonable case can be made that St. Thomas's prohibition pertained only at the global level and therefore at the local level in the creation of particular things he allowed some creaturely causal role in the production of esse, which role may even have been instrumental (ScG III 66, QDP 3, 7). Likewise, the dispositive causal role of human parents can be explained because material being cannot produce immaterial effects (ST 1a 118, 2) and if we suppose the sacraments contain the grace they transmit – as we must theologically – then the creation argument alone does not seem to offer a good reason for explaining why at this stage of his career St. Thomas thought sacramental instrumental causes could only be dispositive causes.

Another area of interest is semantic; ‘potency’ is used in the book in quite distinct senses. Thus on p. 80 ‘natural potency’ is used to mean the power an instrument possesses in virtue of its form, whilst on p. 86 ‘potency’ is used to mean a thing's capacity to be moved. Since the former sense pertains to the order of act and the latter to the order of potency we are left with the same word having senses linked both to the order of act and to the order of potency. Admittedly, context provides the explanation for which sense is intended. Nevertheless, it is slightly awkward and can lead to confusion. This semantic matter leads to a more fundamental concern, though. In any case of instrumental causality, we need to distinguish the proper activity an instrument has in virtue of its form, which belongs to the order of act, and that same instrument's capacity to be moved by a higher cause to achieve its instrumental effect, which belongs to the order of potency. But since the same thing cannot be both in act and potency with respect to the same formality (ScG I c. 13) then in a typical case of instrumental causality the proper effect will pertain to the order of act and the instrument will only need to be moved to achieve its instrumental effect.

This is not the case for sacramental instrumental causality, though. There the proper activity of the sacrament is not enough: further act needs to be introduced. In baptism, for example, no matter how carefully water is brought into contact with the recipient of the sacrament, unless grace is also imparted baptism will not have occurred. The sacrament must be moved both to bring it into contact with the recipient and to impart its instrumental effect and the critical challenge for the Thomist will be to explain how that happens without departing too far from ordinary instrumental causality.

Still, these remarks should not be taken as indicative of any dissatisfaction with the book. Not at all. Like any good book it provokes thought on the part of its readers and it will be interesting and profitable to discover Lynch's further research into these matters.