In the preface of his book, Daniel Garber explains how it developed out of an attempt to ‘update’ his groundbreaking article from 1985, ‘Leibniz and the Foundations of Physics: the Middle Years’ (in K. Okruhlik and J. Brown (eds), The Natural Philosophy of Leibniz, Dordrecht: Reidel) – an attempt that finally turned out to be ‘a massive rethinking of the thesis there’ (vii). Thus, Leibniz. Body, Substance, Monad takes stock of an interpretation of Leibniz's ‘metaphysical physics’ (to use an expression Garber himself invented in the context of Descartes) that has generated an incredible amount of controversy over the last thirty years. Garber outlines the main points of this interpretation as follows: ‘One of my main theses is that in the middle years, Leibniz had not yet come upon the monadological metaphysics that will characterize his later years, the doctrine that everyone knows best from such texts as the “Monadologie”, which, for many readers, is emblematic of Leibniz's thought. Instead, what one finds there is a metaphysics grounded in corporeal substance, extended unities of matter and form’ (xix). While such a statement for the uninitiated may appear fairly innocent, it in fact envelops controversial answers to two questions that have haunted Leibniz research for several decades now, a metaphysical and interpretive one and a methodological one.

The metaphysical and interpretive question concerns the status of ‘corporeal substances’ in Leibniz and which side (if any) of the realist-idealist divide Leibniz should be situated (and when, to the extent that Leibniz's position does not remain fixed). Considerable energy has been deployed by commentators in the last couple of decades in trying to refute or defend Garber's position on this point. There is no need to stress the importance of these debates which have largely defined a whole generation of Leibniz scholars and will continue to do so for some years to come. Let us just shortly recall Garber's main line of argument. Following some of Leibniz's own autobiographical remarks, commentators traditionally consider the date where all the central features of Leibniz's ‘mature philosophy’ were in place to be the year 1686 where he wrote the physical-logical-metaphysical triptych formed by the Brevis Demonstratio Erroris Memorabilis Cartesii, the Generales Inquisitiones de Analysi Notionum et Veritatum, and the Discours de métaphysique. Garber's original interpretive move consists in stressing the artificiality of any such conception of a unified, fixed ‘mature philosophy’ dominating the following forty years of Leibnizian reflections on the nature of bodies and substances. He points to important shifts in Leibniz's central conceptions during that period, arguing that one must speak of a hylemorphic ‘middle period’ distinct both from the more materialistic early Leibniz and from the phenomenalism of the mature monadological metaphysics. During these ‘middle’ years, Leibniz defended a conception of corporeal substances that arguably makes of him a realist, as opposed to the idealist position which for many commentators remains the quintessential Leibnizian metaphysics.

The methodological problem that Garber's study addresses concerns the systematic unity of Leibniz's thought, or more precisely the usefulness of presupposing or even searching for such a systematic unity. Garber's book is not, as one might expect from a Leibniz scholar of his calibre, a monolithic synthesis of ‘Leibniz's Metaphysics’ or ‘Leibniz's Philosophy’. The book he has written is, as he notes, ‘on a very small part of Leibniz's thought’ and ‘not intended as a general introduction’ (i). Moreover, Garber is not a commentator pretending to give definitive answers, even about the ‘small part’ he grabbles with. Indeed, not providing any such definitive answers seems to be a hallmark of Garber's entire approach. His reading brings to light a Leibnizian philosophy full of loose ends, uncompleted arguments and vacillations of mind. We do not get the image of a philosophy, but only of various stages of a philosophical process of reflection. Garber even insists that Leibniz's philosophy was still in a state of development and incompletion at the time of his death: ‘[…] there is no single doctrine that one can say is the Leibnizian solution to the problem […] I don't think that he ever arrived at an answer that fully satisfied him’ (384).

In short, Garber doesn't believe in interpretive projects pretending to ‘capture’ Leibniz in a single, final doctrine. Thus, he writes, he wants to do away with the conception of Leibniz as a ‘doctrinaire philosopher’ that has dominated since Kant and ‘replace him with a deep, subtle, and wide-ranging intellect, constantly thinking and rethinking his position, constantly engaged, who develops and grows, even if, in the end, he never arrives at a position with which he is fully satisfied’ (xvi). Accordingly, Garber provides an account which he characterizes as ‘developmental’ in the sense that he wants ‘to make Leibniz's views intelligible by showing how he came to them’ (xviii). The meaning of Leibniz's philosophy is for Garber identical to the genuine intellectual process through which it came about. Hence, clarifying the development of the doctrine is not just instrumental to understanding it, but is nothing but that understanding itself. In this respect, Garber's work is similar to the work of that other major Leibniz scholar of the last three decades, Michel Fichant, who, in his account of his ‘genetic’ approach to Leibniz, refuses to accept accounts of Leibniz's philosophy in terms of ‘a “system of Leibniz” which ideally is closed off and disconnected from the actual duration of its genesis’ (‘L'invention philosophique’, in Leibniz, Discours de métaphysique, Monadologie, (ed.) M. Fichant, Paris: Gallimard, 2004) 79.

Moreover, the philosophical development or genesis that for Garber just is Leibniz's philosophy is not depicted as a straight line of progression towards a given aim, but includes the twists and turns of Leibniz's reflections, his hesitations, abandoned arguments and constant revisions. Indeed, the aborted projects often explain those that were not aborted. A very good example is Garber's account of how Leibniz in the late 1670s came to understand how his fundamental physical principle – the equipollence of the full cause with the entire effect – should be grounded ‘architecturally’ in metaphysical principles involving the appeal to divine providence through his failed attempt to demonstrate this very same principle geometrically. In this case, it was through realizing that the geometrical demonstration could not be found, i.e. a failed attempt, that Leibniz became aware of the immense benefits for natural theology of affirming this very fact, i.e. that it cannot be demonstrated.

‘Much has been considered and begun, but admittedly nothing has been accomplished and completed’. Leibniz wrote this somewhat gloomy assessment of his own progress in a text from May 1676. For anyone ever confronted with the incredible mass of ambitious but abandoned projects that defines the major part of the Leibnizian corpus, it could however seem an appropriate epitaph for his work as a whole. Many attempts have been made over the years to disprove any such apparently ruinous conclusions wrong by trying to extract a complete and systematic philosophical monument from Leibniz's texts, often somewhat in spite of these texts. It is among Garber's merits to have seen that Leibniz's pessimistic assessment of his own progress is emblematic of a thinker whose incapacity to settle matters and perpetual intellectual unrest was the very engine of its spectacular inventiveness. Leibniz's wavered and changed his mind. But Garber has made of this same wavering and change of mind the very greatness of Leibniz. He has shown the fruitfulness of peeking into the workings of this astonishing spiritual automaton, focusing on how it functioned, how it progressed and how it developed, and not just on what it ‘finally’ produced.