In letters to Christian Garve and Johann Kiesewetter from 1798, Kant claimed that there is a gap in his Critical philosophy. This gap was to be bridged by writing a new work titled Transition from the Metaphysical Foundations of Natural Science to Physics. As most students of Kant know, he never finished or published this work. All we have are drafts of Kant’s transition project, the so-called Opus postumum, contained in volumes 21–2 of the Akademie edition of Kants gesammelte Schriften.

In recent decades, scholarly interest in the Opus postumum has steadily increased. However, it remains one of the least studied works of Kant. There is, moreover, little to no consensus among scholars on the purpose and nature of Kant’s transition project. One reason for this state of affairs is that scholars disagree about the nature of the gap in Kant’s Critical philosophy. In The Post-Critical Kant, Bryan Wesley Hall provides a novel interpretation of this gap and of the transition project. Many commentators think that Kant’s transition project constitutes an extension of the project of the Metaphysical Foundations of Natural Science (1786), or, alternatively, provides a means to remedy certain problems internal to the Metaphysical Foundations. By contrast, Hall argues that the transition project is meant to bridge a gap located in the transcendental part of the metaphysics of nature as presented in the Transcendental Analytic of the Critique of Pure Reason (p. 3). Hence, Kant’s transition project is meant to solve a problem that lies at the heart of Kant’s transcendental philosophy. Since the Opus postumum solves a problem that is central to Kant’s Critical philosophy, Hall argues that ‘Kant’s planned transition project is the post-Critical culmination of his Critical philosophy’ (p. 2). We cannot, in other words, fully understand Kant’s Critical philosophy if we do not study the Opus postumum.

In the first chapter, Hall provides his novel interpretation of the gap in the Critical philosophy. He locates this gap within Kant’s theory of substance, articulated in the Analogies of Experience of the Critique of Pure Reason. How must this gap be understood?

Hall argues that in the First Analogy of the Critique of Pure Reason, Kant’s use of the term ‘substance’ is equivocal. This becomes clear if, following Hall, we consider Kant’s different formulations of the Principle of the First Analogy in the A- and B-editions:

According to Hall, the A-edition formulation suggests that the term ‘substance’ applies to (relatively) enduring empirical objects, i.e. objects that persist through the alteration of their properties (p. 36). On this reading, there are multiple substances and substances are ordinary empirical objects. By contrast, Hall reads the B-edition formulation as suggesting that there exists only one substance that is sempiternal (existing at all times) and omnipresent (existing at all spaces). On the basis of a thorough analysis of the Analogies, Hall argues that both interpretations of the term ‘substance’ have good philosophical and textual grounds. On the one hand, it seems to be a necessary feature of our experience that there are many substances (p. 48). On the other hand, the existence of one sempiternal and omnipresent substance secures ‘the unity of spatiotemporal experience by precluding the possibility of experiencing empty times or spaces’ (p. 44).

The existence of two mutually exclusive interpretations of the term ‘substance’ generates a dilemma, Hall argues (p. 51). This dilemma shows that it is not clear how we should interpret the category of substance within Kant’s philosophy. If the category of substance applies to empirical objects (substances), Kant can explain how we know ordinary empirical objects and their alterations, but cannot demonstrate the unity of spatio-temporal experience. If the category of substance applies to one sempiternal and omnipresent substance, Kant can explain the unity of spatio-temporal experience, but cannot demonstrate how we know ordinary empirical objects and their alterations. Hall thinks this problem constitutes the gap in Kant’s Critical philosophy. To solve this problem, he argues that Kant must interpret the category of substance invoked in the first Critique as applying to multiple empirical objects (substances), while acknowledging that the first Critique fails to demonstrate the existence of one sempiternal and omnipresent substance. This demonstration remains to be given. According to Hall, Kant’s argument for the existence of one sempiternal and omnipresent substance is given in the Ether Deduction of the Opus postumum, which proves a priori the existence of a persistent matter distributed throughout the whole of space (p. 52). The Ether Deduction is thus essential to bridging the gap affecting Kant’s theory of substance.

In the second chapter, Hall sketches the development of the ether theory in Kant’s writings. In his Critical writings, Kant treated the existence of the ether as a (scientific) hypothesis (p. 73). In these writings, the ether is treated as an object whose existence can only be demonstrated a posteriori. By contrast, in the Opus postumum Kant comes to treat the ether as something whose existence can be demonstrated a priori. Hall further describes some of the scientific problems that Kant’s ether theory addresses. In the early sections of the Opus postumum, Kant uses the ether to explain, among other things, (differences between) the density of physical bodies, the cohesion of physical bodies and different states of matter (solid, liquid, gaseous). However, in contrast to some interpreters, such as Michael Friedman and Martin Carrier, Hall does not think that the scientific context within which Kant introduced his speculations about the ether is essential to interpreting the transition project. Following Erich Adickes, Hall argues that ‘Kant’s speculations in empirical physics are highly suspect’ (p. 85). Accordingly, he is ‘skeptical of interpretations of Kant’s transition project and the gap problem that require the ether to play an explanatory role within empirical physics’ (p. 85).

The third chapter provides a (semi-)formal reconstruction of Kant’s Ether Deduction, contained within the Übergang section of the Opus postumum. The Ether Deduction is reconstructed as a transcendental argument that proves that the existence of the ether is a necessary material condition for the unity of experience.

Chapter 4 provides a novel interpretation of Convoluts 10–11 of the Opus postumum. The Ether Deduction showed that the existence of the ether is a necessary material condition for the unity of experience, whereas Kant’s Critical writings established necessary formal conditions for the unity of experience (e.g. space, time, categories). According to Hall, Convoluts 10–11 show that the existence of the ether and the formal conditions of experience, which now also include an a priori concept of the ether, constitute sufficient conditions for the unity of experience (p. 116). The Ether Deduction and Convoluts 10–11 together thus identify necessary and sufficient conditions for the unity of experience.

The fifth and final chapter is one of the richest and one of the most complex chapters of the entire book. Hall thoroughly discusses the problem of affection in Kant’s philosophy, famously introduced by Friedrich Heinrich Jacobi and systematized by Hans Vaihinger, and pays special attention to the theory of affection developed in Convolut 7. Erich Adickes took the Opus postumum to contain arguments for a theory of double affection. Hall rejects this interpretation, noting that there is insufficient textual support for ascribing such a theory to Kant. Rather, the Opus postumum contains a theory of affection which appeals ‘only to affection within the phenomenal world’ (p. 163). Briefly, this theory holds that the moving forces of the ether affect our senses and produce subjective representations, which are subsequently ordered by the subject in accordance with the categories, resulting in cognition of determined phenomenal objects.

The Post-Critical Kant is a major scholarly achievement. It is one of the few books in the English language devoted entirely to the Opus postumum. Moreover, in contrast to many commentators, Hall provides extensive analysis of almost all of the phases of Kant’s transition project. The only part of the Opus postumum that Hall does not discuss is Convolut 1, written from 1800 to 1803, which he takes to differ ‘significantly from the other fascicles’ and which ‘shows distinct signs of Kant’s decreasing mental powers’ (p. 12). The book is well written and concise, and Hall often succeeds in explaining Kant’s famously obscure notes and drafts in a clear manner. His treatment of the Ether Deduction, for example, is one of most accessible presentations of this argument that I have encountered. Finally, Hall also provides a philosophical defence of Kant’s arguments in the transition project and is often successful in showing that these arguments are well motivated and plausible. Hence, both scholars who have studied the Opus postumum and scholars who want to be introduced to Kant’s last projected work will benefit greatly from Hall’s study.

I present one critical remark. Although Hall persuasively argues that the gap in Kant’s philosophy must be located in the Transcendental Analytic, I do not think his interpretation fully explains Kant’s intentions for engaging in his transition project. I was especially uncertain how on Hall’s interpretation, which pays little attention to the Metaphysical Foundations, we can make sense of the title of Kant’s late work: Transition from the Metaphysical Foundations of Natural Science to Physics. This title suggests that Kant aimed to bridge a gap between the Metaphysical Foundations, providing a priori principles of natural science, and empirical physics. The title also suggests that Kant aimed to ground the unity of physics, where the term ‘physics’ can be understood in a traditional Aristotelian sense as a science concerned with nature as a whole. As several commentators have noted, such an interpretation is supported by the fact that Kant, especially in the early stages of the Opus postumum, invokes the ether to provide a unified explanation of phenomena treated within different branches of physics (e.g. chemistry, theories of heat). Hall rejects such an interpretation because he argues that the gap must be located in Kant’s transcendental philosophy. The downside of this interpretation is simply that it is difficult to reconcile with some of the textual evidence provided in the early phases of the transition project. Perhaps scholars should simply view the Opus postumum as a work in progress and conclude that Kant often changed his views on the nature and purpose of the project. Hall agrees that the Opus postumum is unfinished, but nevertheless attempts to provide one unified interpretation of the work.

My remark shows, not surprisingly, that it is difficult to achieve consensus on the goal or purpose of Kant’s transition project. This is not in any way meant to detract from the quality of Hall’s work. The Post-Critical Kant provides a significant advance in the study of Kant’s final drafts and all future scholars working on the Opus postumum will have to take account of Hall’s book.