This is simultaneously a notable contribution to Leibniz scholarship and a less than completely successful book.

The work is divided into three parts: ‘Eucharist and Substance’, ‘Predestination and Necessity’ and ‘Leibniz, the Historian of the Sacred’. There is also a helpful appendix to chapter ii containing ‘A selection of texts on the negotiations’ – although nowhere to be found is ‘the first-ever English translation of the Unvorgreiffliches Bedencken’ advertised on the dust jacket.

In each of the three parts Irena Backus considers the ‘religious’ thought of Leibniz through specific debates or negotiations, many of which have not been widely discussed in previous secondary literature. What is not present in her book, however, is a thesis beyond the very general one: ‘My sole ambition in fact is to show the coexistence of theology and philosophy in Leibniz and to see how the two come together’ (p. 1). The reader who comes to this book expecting an overarching argument about the theological vision of Leibniz or its development will be disappointed.

In her brief introduction, Backus tells us ‘that I am not a philosopher and that the public I have in view consists primarily of historians of ideas, theologians, and historians of religion from graduate students upwards’ (pp. 1–2). Further, she explains that she is writing as a ‘religious historian’ and so ‘the resulting Leibniz is very much a “Leibniz in context”’. She assumes, rather than provides, important background knowledge of Leibniz's own thought and much of the early modern religious context. What Backus gives us in this book is a series of studies of ‘theological’ incidents in Leibniz's life, presented largely discretely but with a fine sense of individual context and the larger religious implications of each.

Backus is a distinguished historian of religion, especially of Calvinism, and she has brought a depth of knowledge about the forms and divisions of early modern Christian thought and its larger medieval background to her analysis of Leibniz's theological involvements. Few in the Leibniz scholarly world could provide this – though it is not unprecedented, for example the work of Antognazza, Rösler and Laerke. In part i she examines the efforts that Leibniz was involved in to reconcile the Lutheran Church with the Roman Catholic and then also with the Calvinist Church. In part ii, she discusses the seventeenth-century debates about predestination and free will involving Hobbes, Bramhall, Locke, Newton and Clarke and others. (In this section she includes a chapter on Augustine as a source for Leibniz in these debates that seems somewhat tangential.) In the third part of the book, Backus turns to consider Leibniz as historian, and specifically as historian of the sacred. Here Leibniz is found in conversation with, or is at least compared to, Huet, Grotius and Newton, among others.

The strength and weakness of this book is that it stays close to the ‘coal face’ of Leibniz's often occasional writings, providing learned and useful analyses of the texts and controversies. She does an impressive job of recreating these theological exchanges and, to lesser and greater degrees, each section addresses gaps in the secondary literature, and deepens and broadens our understanding of Leibniz's religious thought. In all the detail she provides of these theological interchanges, Backus demonstrates her larger goal of showing that Leibniz's religious or theological work cannot be separated from his philosophical.

But this leads us to two fundamental problems with the project outlined.

The first is one that belongs to all efforts to understand and explain Leibniz's thought: there is no Meditations or Ethics, such as Descartes and Spinoza respectively provide, to give us a mature or developed standpoint by which to measure everything else. Leibniz published little of his vast output during his lifetime. The Theodicy (1710) can function only inadequately as an intellectual lodestar, and issues about the development and shape of Leibniz's thought are rife in the scholarship. Backus seems to assume a knowledge of this problem, while at the same time not giving it much weight or the reader much help in her treatment of writings which come from different parts of Leibniz's intellectual career. While it is apparently Leibniz's thought that is central to her work, because she has structured the book topically, any suggestion of development within his thought – something that might have given room for a larger thesis about Leibniz's religious thought – appears only incompletely and in the background.

The second issue is more fundamental to Backus’ whole project, and points to a basic tension at work in it. While she tells us that she is not a philosopher and has ‘no intention to enter into competition’ with ‘present day Anglo-Saxon philosophers’ her basic claim in this book is that it has a necessarily philosophical dimension: that Leibniz's religious thought cannot be separated from his philosophy. Indeed, from the first page of the main body of her text Backus is involved in discussing the very philosophical notion of ‘substance’ to explain Leibniz's account of transubstantiation. Indeed, it belongs to the whole interest of Leibniz's thought as Backus articulates it to look to his metaphysics as precisely a way to resolve deep-seated debates understood in ‘theological’ as opposed to ‘philosophical’ terms.

And this raises what could have been the central question of this work: why did Leibniz think that his philosophy could do this reconciling work? What might the relationship between ‘philosophy’ and ‘theology’ be for Leibniz? Backus touches upon this question, but does not conceptually address it. Indeed, she continually cites the ‘philosophical/theological’ distinction without clearly explaining it. Of course, Leibniz was not alone in thinking that a modern philosophy, independent of revelation, could precisely be the basis for overcoming religious divides or mediating religious truth more adequately than the Hellenic philosophies used in the Middle Ages. For example, Descartes presents his Meditations as providing an independent ground for the Catholic Church's teaching on the existence of God and the immortality of the soul, but as Backus points out his account of substance created significant difficulties for the acceptance of his thought by the Church. Equally the connection of Newton's natural philosophy to his own unorthodox theology and the theology of the Anglican Church has been the subject of important research that Backus cites. What seems distinctive in Leibniz is the extent to which he saw his thought as able to mediate religious thought and even religious difference while preserving the existing forms of religion. What is the basis for this confidence?

Backus makes clear that Leibniz rejected the route of toleration preferred by Bayle and others as a way to allow for religious difference among Christians in Europe. He argued, rather, for an actual unification of those religions on the basis of his philosophy. So, philosophical accounts of substance and of theodicy provide the necessary basis, according to Leibniz, to reconcile Lutherans and Catholics or Lutheran and Reformed Churches about the nature of the eucharist. Equally it is Leibniz's monadology and theodicy that can reconcile predestination and free will. Backus shows how Leibniz's philosophical standpoint informed his reading of Scripture around questions of history, prophecy and the apocalypse, showing the texts as being able to fall within a philosophical theodicy. One of the best things about Backus’ account is that she insists that Leibniz is not simply reshaping his thought for whatever audience is at hand, but that the theological changes over time and context reflect philosophical ones. But what are we then to make of the relationship of theology to philosophy in Leibniz? She tells us that she wants to have us ‘see’ the shared role of theology and philosophy in Leibniz's thought – and she does this admirably. But surely what we need beyond that ‘seeing’ is an explanation of what we see? What is valuable in this scholarly and deeply knowledgeable book is that it raises this most basic question for understanding the religious dimension of Leibniz's thought; but it does not provide an answer to it.