Few other philosophers have shaped the philosophical landscape as Leibniz did. Given the great variety of his thought and the high degree of philosophical innovation that came with his ideas, almost all areas of subsequent philosophy deal with Leibnizian ideas, whether they build on them or strive to reject them. This holds for Kant as well, who was decidedly influenced by the Leibniz-Wolffian school of philosophy that was dominant in Germany in the eighteenth century. This new collection of essays deals with this complicated legacy, the continuity of Leibnizian thought in Kant’s philosophy and the ways in which he breaks with it. It contains twelve papers by established authors on a variety of topics that were presented in earlier forms at a joint conference of the Leibniz Society of North America with the North American Kant Society in 2009. Not all contributions focus on both thinkers equally or take their relation into focus. Some rather zoom in on one thinker with the other being reduced to an afterthought. However, the systematic issues that are discussed are deeply relevant to both philosophers. Hence, even the papers which might be considered a bit more one-sided will still contribute important ideas to the overall research debate concerning the Leibniz–Kant relation.

Given the wide range of topics that are discussed by Leibniz and Kant, a comprehensive treatment of all the ways in which these authors relate to each other cannot be developed within a single book, so a selection is inevitable. Speaking broadly, the volume is more interested in traditionally important epistemological or metaphysical issues. The question as to how both authors conceive of reality and appearances pervades many texts, as this is relevant to the assessment of their respective forms of realism and idealism. Notions of perception, cognition, force and space play crucial roles here, as does the relation of God to his creation.

In the first chapter, which also functions as an introduction, Brandon Look sketches the difficult relation between these thinkers: while Kant often praised and criticized Leibniz, it is not always clear how much he had read or understood his texts and where he might have seen the differences between Leibniz and Wolff. Look also provides a broad and helpful sketch of the intellectual environment in which Leibniz-Wolffian thoughts were taught, transformed and disseminated during Kant’s times, so influences might have been even more indirect. In her contribution, Ursula Goldenbaum discusses alleged Wolffian influences on the early Kant’s conception of force. With a remarkably erudite discussion of the subtle conceptions of forces in the early seventeenth century she argues that Kant largely follows Pietist ideas instead of the Wolffians, as he is more interested in solving the question how the soul can act on bodies than in finding the true measure of force. In the third chapter, Erik Watkins discusses Kant’s break with rationalism, in the pre-Critical and Critical period. Presumably informed by Crusius’ anti-rationalist work, Kant emphasizes the distinction between existence and essence and the respective faculties for representing either. However, Leibniz also distinguishes between essence and existence as different in kind, as possible worlds (wherein essences are conceived) are conceptually contained in the mind of God, while the actual world is distinguished from merely possible worlds by means of the divine act of creation and the substantial force (or monadic appetite) that constitutes the very fabric of reality.Footnote ¹ Here, Leibniz is a late follower of the Platonic and Neoplatonic tradition of distinguishing between ideas and matter. While Leibniz did not sufficiently distinguish the cognitive faculties to perceive essences or ideas and phenomena, I am not entirely convinced that we need to look to Crusius to identify the origins of Kant’s distinction between existence and essence.

In the fourth chapter, Donald Rutherford provides a new take on Leibniz’s conception of space. Usually, Leibniz has been ascribed relationalist and idealist views on space, i.e. the ideas that space is constituted by relations and that space is mind-dependent. In contrast, Rutherford argues that space needs to be represented as a whole, not as a compositum of primitive relations. In the next chapter, Alison Laywine argues for reading Kant’s notion of nature as denoting something that has a world-like character, and she also looks at Kant’s take on Leibniz in the Transcendental Deduction. Here, Kant provides arguments for his claim that the categories prescribe laws to nature, but he does so in a manner that echoes his earlier, Leibnizian take that God’s relation to the world acts as a paradigm for our human understanding of the world. Another similarity and possible influence is discussed by Nicholas Stang. Both Leibniz and the pre-Critical Kant discuss ideas about the merely phenomenal status of matter and the way matter ontologically results from an organized aggregate of simple substances. Stang emphasizes the subtle vocabulary that Leibniz employs to express different types of ontological dependency, such as ‘resulting from’, ‘being required by’ or different senses of ‘being in’.Footnote ² He uses this to provide more nuanced readings of both Leibniz’s substance idealism and matter idealism, as discussed in important works by Rutherford and Robert M. Adams, and he compares this reading to the respective forms of idealism present throughout Kant’s early works. For both Leibniz and Kant, bodies are more than just ‘mere phenomena’, albeit Leibniz has a more subtle vocabulary at hand to express ontological dependencies between substance and phenomenal body.

One of the major criticisms that Kant employs against Leibniz is that he sees the difference between sensible and intellectual representations as a difference of logical representations, and that Leibniz thinks that intellectual representations are fundamental, while missing the equal importance of sensible cognition. This is the topic of Anja Jauernig’s chapter in this volume. She first strives to provide us with a detailed reconstruction of this Kantian criticism, as Kant rejects the Leibnizian criterion of distinctness. Both types of representation are traced back to the different stems of cognition, which are active and passive, respectively. This leads Kant to affirm the importance of sensible representation, while accusing Leibniz of downplaying it. This accusation can be defused by distinguishing different senses of ‘sensibility’ in Leibniz. Kant’s claim that Leibniz thinks that we can experience things as they are in themselves is also shown to be false. This is an important point, as it indicates that Kant might also misunderstand Leibniz’s nuanced and complicated take on the relation between appearances and their underlying substances.

The eighth chapter by Martha Brandt Bolton discusses Kant’s Amphiboly as a critique of Leibniz in a similar vein, inquiring as to how Kant construes Leibniz’s views of spatial cognition and sense perception in a problematic way. Bolton also argues that Leibniz’s own theory of cognition is much closer to Kant than he acknowledges and that one should refrain from reading the first Critique as a definite refutation of Leibnizian ideas of representation. Important differences between both philosophical systems are fleshed out by Paul Guyer in his contribution. Kant is occasionally enticed to read Leibniz’s conception of harmony between the kingdoms of nature and grace as an anticipation of his own distinction between nature and freedom and their respective conjunction in the realization of the kingdom of ends in nature. But Guyer argues that these two realms are much more closely connected, even identical in Kant, while Leibniz needs to conceive of them in strict separation. Both share the idea that inert matter needs to be opposed to the conception of organisms, which requires the adoption of a teleological view of nature instead of a merely mechanical one. However, the end of this teleology is conceived very differently: while Leibniz adheres to the theological idea of a human community with God, Kant employs rather the regulative idea that only pure reason can posit the highest good.

This brings us to the last three chapters that deal with the philosophical theologies of both thinkers. Desmond Hogan discusses the relation between divine and secondary causation. He situates Kant within a framework between occasionalism, conservationism and theories of concurrence. Hogan looks at early and later writings by Kant and then concludes that Kant conceives of all creatures as ontologically dependent on God and that there is a cooperation or concurrence of God with all worldly secondary causes. However, this concurrence entails a tension with his own rather libertarian theory of freedom, wherein we can conceive of ourselves as proper causes, undetermined by external factors. In the next chapter, Patrick Kain discusses Kant’s conception of the free divine will, which might be inspired by Leibniz. In contrast to our human will, the holy will is determined by reason alone. Now one might wonder whether such a purely rational determination of the will would be different from mechanical determination, as both introduce necessity into the will. Kain argues that mechanical determination might be similar to moral necessity in the sense that neither allows for the possibility to do otherwise, but mechanism fails to provide normative guidance, which is precisely the point of the determination of the will through reason. Finally, Andrew Chignell examines Leibniz’s and Kant’s takes on empirical miracles. An empirical miracle entails a disturbance of the natural course of events by a supernatural force, as opposed to other miracles such as the very act of creation itself. But what exactly do miracles conflict with? Empirical miracles will in most cases conflict with nature’s powers, or with subordinate principles or maxims, or they may appear as products of supernatural concurrence with natural activity. Furthermore, Leibniz distinguishes between nature and essence, between that which is realized by means of active forces in a narrower sense and that which is conceived within the idea or complete notion of an individual in a wider sense. This nuanced distinction allows Leibniz to conceive of certain events as virtually embedded in the essence of individuals, but not in their natures; and as our knowledge of nature is restricted to natures, miracles appear unnatural or supernatural to us, albeit without requiring an actual divine intervention in time. Hence, miracles appear to us at least partly because of our ignorance of the general order of nature. Kant seems to hold roughly the same view, albeit he can do so without the many Leibnizian metaphysical commitments.

This review obviously cannot provide a detailed examination of all the subtleties discussed in this collection, so I will restrict myself to some more general remarks. This timely book fits well with current themes in Kant scholarship, which is increasingly interested in contextualizing Kant within a framework of debates between his contemporaries and predecessors, instead of restricting itself to analysing and reconstructing the logical structure of his arguments. It also complements other recent scholarship on Leibniz and Kant, such as Justin Smith’s (Reference Smith2017) take on Leibnizian sources of Kant’s concept of species, Daniel Garber’s and Beatrice Longuenesse’s (Reference Garber and Longuenesse2008) collection on Kant’s early modern predecessors and, most recently, Lloyd Strickland’s and Julia Weckend’s (Reference Strickland and Weckend2019) anthology on the reception of Leibniz. I also appreciate the occasional inclusion of German and French sources, both primary and secondary, instead of a narrower focus on exclusively Anglophone scholarship. The broad thematical scope provides many arguments and insights that will be of interest to other scholars of the early modern period, even to those who do not focus their work on Leibniz or Kant: either because many other authors are discussed in their relationship to either Kant or Leibniz; or simply because the masterful examination and discussion of two major philosophical ‘landmarks’ will be helpful to anyone navigating the seas of early modern philosophy.