De Boer’s thought-provoking book participates in a wider tendency of recent historiography to inquire into Kant’s historical sources. Wolff and his disciples, who have been suffering from a poor philosophical reputation for centuries, have finally begun to spark a wider interest, as can be seen from the publication of Wolff’s Gesammelte Werke by Jean École and other critical editions, the first International Wolff Kongress in 2004, the Wolff Handbuch recently edited by Robert Theis and Alexander Aichele, various research groups in Europe, the United States and Canada, and the foundation of the Internationale Christian Wolff Gesellschaft in Halle. One may also mention many important translations, edited volumes, the collection directed by Gideon Stiening and Frank Grunert on eighteenth-century thinkers, and studies on Wolff and the Wolffians by Paola Rumore, Clemens Schwaiger, Jean-Paul Paccioni, Corey Dyck and many others.

So, undoubtedly, there have been a number of forerunners to De Boer’s book. But De Boer’s merit is to have formulated a hypothesis that concerns the very core of Kantian philosophy. She proposes to read Kant’s first Critique as a reform or rebirth of Wolff’s metaphysics, arguing that, notwithstanding the critical revolution, Kant himself remained at least to some extent a Wolffian and just undertook a kind of ‘detour’ in the Critique, as he puts it in a letter to Kästner dated August 1790. This perspective is indeed new and productive. It sets an end to a long tradition that has employed Wolff merely as a negative foil, picturing him as a one-eyed philosopher who was bound by too much conceptual armour to be able to overcome his pre-critical prejudices.

Instead of repeating these old topoi, De Boer takes the opposite standpoint. She presents Kant as pursuing Wolff’s project in important respects, thereby attempting to make better sense of both Wolff and Kant, which already from a purely methodological standpoint seems to be a far more promising strategy. To spell out such a hypothesis in detail is a challenging task, requiring a thorough knowledge of Wolff, Kant and the many technical debates about the first Critique. De Boer tackles the sources and the huge amount of secondary literature, sparing no effort. The result is a book with a clear focus and a highly convincing main argument, which at the same time develops various further arguments, raises millions of questions and sets a challenging research agenda for the years to come.

I must admit that I was quite convinced by the main argument from the outset. But I do remain intrigued about some of De Boer’s arguments. In what follows I will focus on four questions that her research has inspired me to ask.

First, in the second preface of the Critique of Pure Reason, Kant adopts a both metaphysical and epistemological approach when he mentions ‘the plan that the Critique prescribes i.e. the future system of metaphysics and its method’, as well as Wolff’s merit as the greatest among all dogmatic philosophers who ‘would have been the most suited to bring … metaphysics into the condition of a science, … if only it had occurred to him to prepare the field for it by a critique of the organ, namely of pure reason itself’ (Bxxxvi; trans., as throughout, from the Cambridge Edition).

De Boer prioritizes the metaphysical approach over the epistemological one, associating the former with several metaphysical interpretations defended in the 1910s and 1920s, including Heidegger’s, and the latter with neo-Kantian and analytical readings by Cohen, Strawson and others. She contends that her own interpretation is closer to the metaphysical approach than to the epistemological one. My question is: does one really need to dissociate these two approaches? Would it not be more productive to consider their intricate relationship in its historical context, if one’s ambition is to ‘read Kant in view of the past to which he responded’ (De Boer Reference De Boer2020: 15; hereafter referenced simply by page number)? To put the same question differently: are the metaphysical and logical or epistemological problems not intrinsically related for both Wolff and Kant? And is not Kant’s transformation of metaphysics part of an epistemological program and debate on method that characterizes the early modern age and the Wolffian school?

Let us have a glance at one of Wolff’s shorter essays that De Boer, despite her very careful reading of key Wolffian treatises, does not mention. Interestingly, the format of these shorter essays allows Wolff a greater freedom to position himself in various debates. I therefore strongly recommend them, in particular the essay De notionibus directricibus et genuine usu philosophiae primae from 1729 (Wolff Reference Wolff2001). In this essay, Wolff clearly announces his project, namely, to transform what he calls first philosophy, i.e. ontology, into an architectonic science and into a set of directing or guiding notions according to a systematic order, which in his view had been missing in the older Schulphilosophie or scholasticism (De Boer alludes to this on p. 22). If cast into a systematic mould, these directing notions have a methodological value, according to Wolff. They can serve as the first elements and rules for the discovery of as yet unknown truths. These directive notions, Wolff asserts, show ‘where I must direct my thoughts so that I will not go astray’ and ‘kindle a light so that I can see my way’ (Wolff Reference Wolff2001: §3; see also Buchenau Reference Buchenau2011). According to a second and equally fascinating essay, titled De differentia intellectus systematici et non systematici (Wolff Reference Wolff2019), systematizing basically means connecting. The project of systematizing ontology that Wolff envisioned sets out from common notions, which according to Wolff we all empirically know and share.

I take these essays to show that Wolff shifted the focus from mathematics and logic to metaphysics within the debate on method, without thereby separating the two types of thought. In my view, it would be worth the effort to inquire more deeply than De Boer does into Wolff’s conception of method, his ideas of directive notions, architectonic and systematicity, and the broader Cartesian and post-Cartesian debate to which he contributed, as well as into Kant’s debt to Wolff in this regard. Does not Kant directly take up this Wolffian project by demanding that metaphysics take ‘the secure path of a science’ by drafting, as he puts it, a ‘catalogue’ or ‘inventory’ of all pure concepts of the understanding, and by calling his critique a ‘propaedeutic’ to such a metaphysics, a ‘discipline’ and an ‘organon’ (CPR, Bxxxvi)? Does not Kant follow Wolff’s invitation to further develop the plan that Wolff sketched?

In short, how does De Boer conceive of the relationship between the metaphysical and epistemological or logical approaches in Wolff and Kant’s philosophy? And what light does Kant’s epistemological or gnoseological transformation of metaphysics shed on his very notion of metaphysics, its definition and scope and its higher aims, which, as De Boer suggests, are moral ones?

My second question follows from the first and concerns the narrative that De Boer proposes in her first two chapters. These chapters offer a highly instructive and careful reconstruction of Kant’s reading of Wolff in the 1760s and 1770s and include many valuable quotations. Mentioning Kant’s debt to both Wolff’s gnoseological approach to ontology and his ambition to dispel scholastic obscurity, De Boer attempts to identify the different layers of Kant’s reflections. She explains how Kant’s development of a critical viewpoint entailed a different systematization of the directive notions treated in Wolffian ontology. She emphasizes in particular Kant’s novel distinction between empirical and intellectual notions, arguing that in the Inaugural Dissertation Kant came to distinguish between ‘two complementary usages of intellectual concepts, either by relying on sensibility, or by abstaining from the latter’, i.e. by expelling sensibility from its domain (p. 62). She also draws a distinction between Kant’s first-order investigation into the principles constitutive of any cognition of objects and his second-order investigation into the conditions of possibility of metaphysics (pp. 74 and 214).

However, my perspective on the relationship – and the disagreements – between Wolff and Kant differs from De Boer’s on various points. From my viewpoint, a first disagreement between Kant and Wolff concerns their respective methods. In this regard, Kant’s Inquiry Concerning the Distinctness of the Principles of Natural Theology and Morality from 1764 deserves closer attention. In this treatise, which deals with the methodological tools for attaining distinctness and certainty in mathematics and philosophy, Kant’s declared ambition is to clear up Wolffian misunderstandings, overcome erroneous first notions and habits, and announce a new architectonic science which is no longer simply an ontology. As I see it, however, Kant in this text does not give up the Wolffian idea, put forward in the Preliminary Discourse, of a set of common empirical notions which serve as a starting point for both the mathematician and the philosopher. On this reading and contrary to De Boer’s claim, Kant’s philosophical inquiry into first grounds or principles and pure notions remains concerned with common empirical notions at some level.

I furthermore believe that, in order to explain Kant’s disagreements with Wolff, it is important to take into account Kant’s combination of dogmatic and sceptical perspectives, which in his view is required to secure the foundations of a metaphysical system and the constitution of a common empirical world. It is this combination of perspectives that allows him to overcome the partiality of the Wolffian viewpoint and establish a unique philosophy that replaces the various philosophical systems and schools that existed at the time.

My third question concerns the Wolffians. Do Wolff’s students not deserve greater attention in De Boer’s attempt at identifying the various layers of Kant’s progressive systematic ordering of notions? One may mention Baumgarten, whose Metaphysica De Boer does discuss, or Crusius, although I am not sure how central he is. But one may also think of Sulzer or Mendelssohn, the latter of whom figures as a direct interlocutor in the Paralogisms. While Wolff introduced the idea of directive notions, he neither endorsed the same doctrine of the faculties of the mind as Mendelssohn nor drew a distinction between ideas and concepts. As a matter of fact, Kant’s claim that ideas emerge from the effort of the human mind to grasp a totality, as well as his very distinction between ideas and concepts (and categories), seem to be linked to a Platonic perspective that emerged within Wolffianism, in particular in the debate on the vocation of man and Mendelssohn’s Phaedon, which Kant seems to have known well (see Reich Reference Reich1935). So there seems to be at least one missing link here.

Finally, if Kant settled part of his dispute with Wolff in the first Critique, after already having spent much effort to refute him in his pre-critical writings, he was not yet finished in 1781. Continuing to use Wolffian textbooks in his lectures, Kant constantly returned to unresolved issues in later writings. He does so in the Groundwork and the second Critique, which he presents as an improved version of Wolffian universal practical philosophy, and in the third Critique, in which he treats teleology, i.e. one of the disciplines invented by Wolff. Now I do not want to suggest that De Boer should or could have treated these topics in a single book. On the contrary, I think she made the right choice by focusing on a single question, namely, Kant’s engagement with Wolff’s metaphysics in the first Critique. Nonetheless, I would like to stress that Kant already had a certain philosophical agenda in 1781, the contours of which are incipient in the first Critique, and which De Boer only hints at in her final chapter (p. 248).

I mention this point because I believe that we sometimes tend to lose sight of the larger perspective in discussions of technical details or what might in the end be mainly terminological similarities. The starting point ought to remain the big questions and thoughts. As Wolff puts it, human beings, by their very nature and contemplative tendency, have kept asking questions concerning God, the world, the soul of men and all things, as stated in the title of his German Metaphysics. What I would like to suggest, to conclude, is that De Boer may have not yet fully measured how important her Wolffian perspectives on Kant are. I believe they set up a programme for the new kind of Kant scholarship that has begun to emerge, one that allows us to bring into focus again the deeper stakes, the bigger picture and the philosophical projects themselves.