BRYAN HALL | The Post-Critical Kant: Understanding the Critical Philosophy through the Opus Postumum | Routledge 2015


By Hein van den Berg 

In a letter to Christian Garve, dated September 21st, 1798, Kant laments that his philosophy is not completed:

I see before me the unpaid bill of my uncompleted philosophy, even while I am aware that philosophy, both as regards its means and its ends, is capable of completion. It is a pain like that of Tantalus though not a hopeless pain. The project on which I am now working concerns the ‘Transition from the metaphysical foundations of natural science to physics’. It must be completed, or else a gap will remain in the critical philosophy. (Br, AA 12:257)

Kant never completed his transition project. The drafts he left are contained in volumes 21 and 22 of the Akademie edition of Kants gesammelte Schriften, the so-called Opus postumum (see van den Berg 2016:329).

In his The Post-Critical Kant: Understanding the Critical Philosophy through the Opus postumum, Bryan Hall provides a highly innovative and original reading of the gap in Kant’s Critical philosophy and of the aims and nature of his transition project. My aim in this review is to start an open discussion on how we should continue research on the Opus postumum. In the first section I provide some general remarks on Hall’s book and the Opus postumum. The second section summarises Hall’s interpretation of the gap in Kant’s Critical philosophy. In the third section I present my own views on the aims of Kant’s transition project. This sets the stage for the fourth and final section, in which I discus Hall’s methodological principles for studying the Opus postumum and, hopefully, initiate debate on how to continue Hall’s important research.

1. The Post-Critical Kant

I would like to start by praising the accomplishments of Bryan Hall. As I have said elsewhere (van den Berg 2016), I think The Post-Critical Kant is a major philosophical achievement. As I wrote there, the book “provides a significant advance in the study of Kant’s final drafts” and I think that “all future scholars working on the Opus postumum will have to take account of Hall’s book” (2016:333). To appreciate the quality of Hall’s study, a few things must be said on the Opus postumum (OP). The most important thing to note is that it is not a finished work. As Hall himself says: “It is extremely important to judge OP not as a completed book, but rather as, at best, a working draft of something left uncompleted” (p. 5). This statement is very true and captures the core difficulty that interpreters of the OP face. However, it does not convey the extreme difficulties and intense frustrations that occupy students of the OP. In the following, I shall outline some of these difficulties and frustrations.

Most students of Kant know that his texts are difficult to read and understand. It takes multiple years of training to acquire the skills to do proper research on Kant. Now consider the OP. The text does not have proper chapters or section headings. The material we read is not ordered chronologically. Kant often writes multiple drafts of arguments. As a result, he often simply writes down different arguments, and his statements and arguments often contradict each other. Given these difficulties, it is extremely demanding to understand what Kant is actually trying to accomplish in the OP. When you start reading this work, you often simply do not understand what Kant is arguing for.

The student of Kant’s final reflections is faced with more difficulties. The transition project, the drafts of which are contained in what we call the Opus postumum, builds upon all of Kant’s Critical writings. Without, at the very least, having a very thorough understanding of the Critique of Pure Reason (1781–87), the Metaphysical Foundations of Natural Science (1786), and the Critique of Judgment (1790), one will understand very little of the contents of the OP. Moreover, throughout the OP Kant reacts to philosophical and scientific developments in the 1780s and the 1790s (See e.g. Beiser 2002; Friedman 1992; Förster 2000; Onnasch 2009). A full understanding of the OP presupposes a thorough acquaintance with these developments. Finally, the reflections contained in the OP span multiple years, i.e., (roughly) from 1796 to 1803,[1] and it is likely that Kant changed his mind on the purported goal of his transition project (possibly more than once). Given all these interpretative difficulties, it is no surprise that there is little consensus among scholars on the purpose and content of the OP.

The Post-Critical Kant constitutes a significant advance in research on the OP because (i) Hall provides a novel interpretation of the aim of Kant’s transition project that relates the OP to problems within the Critique of Pure Reason (see below); (ii) it is one of the few existing studies that is completely devoted to the OP. With the exception of Convolut 1, Hall describes all the phases of Kant’s transition project (see van den Berg 2016); and (iii) Hall’s book is one of the most accessible studies written on the OP. This is a major accomplishment. Because of the complexities of the OP and because this work presupposes knowledge of almost all of Kant’s previous major writings, many studies on OP are quite complex. Hall, however, uses tools and methods that significantly enhance the readability of the text. He provides, for example, a (semi) formal reconstruction of Kant’s Ether Deduction, which results in a very clear presentation of this famously difficult argument, and illustrates the structure of Kant’s arguments with diagrams. Graduate students or junior researchers who wish to begin to study the OP can, I think, profit a lot if they start by reading Hall’s book.[2]

2. The Gap in the Critical Philosophy

The Post-Critical Kant provides a novel and original interpretation of the aims of Kant’s transition project. What is distinctive of Hall’s book is that he thinks the transition project is meant to solve a problem within Kant’s Critique of Pure Reason. According to Hall, the theory of substance developed in the Analogies of Experience generates a dilemma for Kant. In a nutshell, Hall argues that in the Analogies Kant introduces two (mutually incompatible) concepts of substance. The first concept of substance refers to objects that persist through the alteration of their properties (p. 36; cf. van den Berg 2016:330).[3] If we accept this reading, there exist multiple substances. On the second reading, there is only one substance that exists at all times and at all spaces (see van den Berg 2016:330). Kant needs the first concept of substance to explain how we know ordinary empirical objects and their alterations, whereas he needs the second concept to explain the unity of spatio-temporal experience (ibid.). This problem, according to Hall, constitutes the infamous gap that Kant locates in his Critical philosophy, and which the transition project was meant to solve. As Hall puts the point in his conclusion:

[…] Kant appears to locate the gap in the “transcendental part” of his philosophy of nature. In other words, one should look for the gap in the Transcendental Analytic of CPR. In Chapter One, I argued that this gap can be located in the Analogies of Experience in CPR, where Kant faces a dilemma given the fact that he has only one concept of substance (the a priori category) which is insufficient to disambiguate between the two different and mutually irreducible conceptions of substance he makes use of in the Analogies. (p. 207)

According to Hall, Kant eventually interprets the category of substance employed in the First Critique as applying to multiple substances, and acknowledges that the First Critique did not demonstrate the existence of one sempiternal and omnipresent substance (see van den Berg 2016:330). This demonstration is given by the Ether deduction in the Opus postumum, which is thus essential to bridge the gap in his Critical philosophy.

It is important to note that, in contrast to many interpreters (including myself), Hall does not think that Kant’s considerations on the developments within the sciences, or even Kant’s views on the status of the Metaphysical Foundations of Natural Science (1786), are central to understanding the aims of his transition project. This becomes clear in comments such as the following:

As commentators like Adickes have noted, however, Kant’s speculations in empirical physics are highly suspect. Given these problems […], I am equally 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)

In the next section, I shall provide some reasons for why, in contrast to Hall, I think considerations considering empirical physics and the sciences are important to understand the purpose of Kant’s transition project. My aim in doing so is not to show that Hall’s interpretation of the gap in Kant’s Critical philosophy is incorrect. Rather, I hope to use this difference of opinion to discuss some methodological issues that arise when studying the OP in the final section.

3. The Opus postumum and the Unity of Physics

In my own study of the OP (van den Berg 2014), I have argued, building mainly on the work of Hoppe (1969), Friedman (1992), Falkenburg (2000), Pollok (2001), and Emundts (2004), that the main purpose of the OP is to secure the systematic unity of physics. Here, it is important to take the term ‘physics’ in a traditional Aristotelian sense, referring to the science of nature as a whole. On the basis of a study of Kant’s published writings, the OP, and various eighteenth-century handbooks on physics, I have argued that physics in the eighteenth century was often conceived of as comprising:[4]

  1. Physica generalis, the a priori study of universal properties of matter (the method employed in this part is synthetic). Physica generalis contains metaphysical definitions, mathematical descriptions, and discussion of mechanics and the theory of gravitation. Topics discussed include (a) ‘extension’, (b) ‘impenetrability’, (c) ‘divisibility’, (d) ’movability’, and (e) ‘force’ (Kant’s Metaphysical Foundations roughly corresponds to Physica generalis. However, Kant takes great pride in strictly distinguishing the metaphysical from the mathematical foundations of natural science, and reshuffles some traditional distinctions).
  2. Physica specialis, the a posteriori study of particular properties of bodies (the method employed in this part is analytic). In this part, we consider specific types of bodies. It contains discussion of, e.g., (1) fluid bodies (air, heat and light, electricity, etc.), (2) solid bodies (elastic solids and brittle solids), (3) dissolution, precipitation and various topics pertaining to chemistry.
  3. Physica specialissima (sometimes treated within physica specialis). This part contains discussion of various topics pertaining to natural history, i.e., mineralogy, botany and zoology. Hence, what we now call biology is part of physics (van den Berg 2014:160–3).[5]

An important feature of eighteenth-century textbooks on physics was the debate on how these different parts of physics can constitute a systematic unity (see van den Berg 2014). This question, in my view, is also the central issue addressed in the OP. For example, in the Oktaventwurf (1796), which is often regarded as one of the earliest parts of Kant’s transition project, we read:

This physics is, in turn, divided into general physics (physica generalis), which expresses only the properties of matter in outer objects of experience, and that (physica specialis) which attends to bodies formed from this matter in a particular way, and which draws up a system of them—for example, regarding the difference between organic and inorganic bodies.
If it is introduced by no relationship, the progress from one system to the other is not a transition (transitus) but a leap (saltus), which entirely destroys what is systematic, and, hence, what is scientific in a doctrine; it cannot be tolerated in a philosophy such as physics ought to be, for the fragmentary treatment of its object carries with it no connection of concepts and does not amount to a whole even for memory. (OP, AA 21:407)

For a full explanation of these and other quotes, and for a full defence of my views, the reader is referred to van den Berg (2014). However, I hope to have provided some plausibility to the claim that the problem of the systematic unity of physics is central to Kant’s OP.

To strengthen this case, let us look at the contents of the Oktaventwurf. The majority of topics discussed in the Oktaventwurf pertain to natural science, and more specifically to what was called physica specialis. Kant discusses, among others, (i) density differences among material bodies; (ii) cohesion; (iii) dissolution, precipitation, and crystallisation; (iv) theories of heat; (v) electricity and magnetism; (vi) the states of matter; (vii) the different types of air (Luftarten); (viii) light; (ix) droplet-shaped fluidity; and (x) the behaviour of liquids in capillary tubes. Importantly, Kant often uses the ether to explain various of the phenomena listed in (i)–(x) (van den Berg 2014, p. 179. Cf. Adickes 1920). To me, these facts sufficiently show that topics pertaining to natural science are essential to understanding the aims of Kant’s transition project. For these and other reasons, I simply take Kant’s “Transition from the metaphysical foundations of natural science to physics” to be a project that aims to connect the a priori and the empirical parts of physics. This is necessary in order to ground physics as a systematic and unified science.

The account I have given above is of course insufficient to explain the gap that Kant locates in his Critical philosophy. This would require explaining why establishing the unity of physics is necessary for Kant’s Critical philosophy, i.e., why there would be a gap in Kant’s Critical philosophy if the unity of physics is not established (see, for an attempt, Friedman 1992). Simply put: I have not given an account of the gap in Kant’s Critical philosophy. However, my account does give some insight into what Kant is doing in the early parts of the OP. This suffices to introduce the main part of my review: a discussion of some of the methodological issues involved in studying the OP and of how we might continue this research.

4. Methodology and the Future of Research on the OP

In the Introduction to his work, Hall formulates methodological principles that guide his interpretation of the OP. These are:

[i] A good interpretation should be maximally consistent with the text.

This involves, Hall argues, not

only taking into account OP, but Kant’s Critical-era texts as well.

[ii] A good interpretation should make Kant maximally consistent with himself.

This involves making sense of conflicting passages within the OP. In addition, one

should try to make the post-Critical Kant as consistent as possible with the Critical Kant.

[iii] A good interpretation should be philosophically plausible.

[iv] A good interpretation should reflect Kant’s intent for OP. (pp. 5–6)

It is a testament to Hall’s clarity and precision that he makes his methodological principles explicit. Moreover, the principles are reasonable and Hall attempts to follow them throughout his work. However, given the highly unfinished nature of the OP, I was not always sure whether all of these principles were entirely appropriate for studying the OP. In the following, I present some of my worries.

Ad (i). I think this principle is unproblematic and should be followed by any interpreter of a historical text. The question, of course, is which texts we take to be central. Hall has provided an interpretation of Kant’s gap that nicely relates the Critique of Pure Reason to the OP, and makes sense of the claim that there is a gap within the Critical philosophy. However, as the juxtaposition of Hall’s views with those of myself make clear, Kant’s Metaphysical Foundations of Natural Science plays a relatively minor role within Hall’s interpretation. Moreover, as we have seen, it is not quite clear to me how Hall’s interpretation of the gap helps us understand the many passages in the OP that seem to be devoted purely to scientific topics (see my remarks on the Oktaventwurf above).

Ad (ii). In general, I think this principle is rational and should be adopted by historians. However, I was not quite sure whether it is appropriate for the OP. As Quentin Skinner has pointed out, historians are sometimes too prone to look for consistency. We should always be open to the possibility that philosophers in the past have been inconsistent (Skinner 1969). I think this certainly applies to the OP. It is an unfinished and unpublished work, in which Kant often seems to change his mind. Why not simply embrace the inconsistencies? Hall acknowledges the unfinished nature of the OP, but nevertheless tries to give a unified explanation of the entire work (see van den Berg 2016). Why did Hall adopt this approach?

Ad (iii). I was most uncertain of the utility of this principle in historical research. For who decides what is philosophically plausible? As we have seen, Hall employs this principle to reject interpretations of Kant’s transition project that take Kant’s engagement with the sciences to be central to this project. The reason Hall gives is that Kant’s speculations on physics are suspect. I agree that Kant’s speculations are suspect, but, as we have seen above, I think they are nevertheless central to the transition project. Many contemporary philosophers would reject the entire idea of providing a priori foundations of natural science or the idea of establishing the unity of physics. Today, such projects are arguably philosophically suspect. Yet everyone would agree that these projects were essential to Kant’s philosophy, e.g. in his Metaphysical Foundations of 1786.

Ad (iv) I do not personally know any historian who would disagree with this maxim. It is one of the great advantages of Hall’s interpretation that he has provided an account of the OP that satisfies this maxim.

I hope my remarks on maxims (i)–(iv) provide the basis for some discussion of how we should continue the research on the OP. As I have said, I think The Post-Critical Kant is a major advance in scholarship, but more work needs to be done. Here is my dream: I would like the edition of the OP that we have to be republished, chronologically, with fragments that are still missing included. I would like somebody, perhaps with digital means, to simply provide a list of topics that are discussed in every fragment of the OP. This would provide us with an overview of the content of the entire work that we now often lack. This could then, hopefully, be a basis for more commentary-like studies on the OP that still need to be done. Having said all of this, I would like to conclude by saying that Hall did a great job in providing a stepping-stone to a better study and understanding of the OP.

Acknowledgement: I would like to thank Jeroen Smid for comments on an earlier version of this article.

Invited: 5 June 2016; received: 31 August 2016.


[1] Here I follow Adickes’ chronology of Kant’s Nachlasswerk, contained at the end of volume 22 of the Akademie edition of Kants gesammelte Schriften

[2] Of course, there are many other works that are essential to understanding the OP. These include (among others): Basile (2013), Beiser (2002), Edwards (2000), Emundts (2004), Förster (2000), Friedman (1992), Gloy (1976), Hoppe (1969), Mathieu (1989), Tuschling (1971) and Westphal (2004).

[3] In the following, I occasionally lift (parts of) sentences from my review of Hall (van den Berg 2016). 

[4] The clearest exposition of what follows can be found in Eberhard’s Erste Gründe der Naturlehre (1774). See also Pollok (2001:516–18). My description of what follows is best understood as an account of Eberhard, although similar descriptions can also be found in other authors. It should be noted, however, that there was a lot of debate among philosophers and physicists on the nature of physics and on whether physics can constitute a unity. In short: the notion of ‘physics’ was in flux. For more on these debates, see van den Berg (2014). In the following, I excerpt small sections and occasionally lift sentences from van den Berg (2014), Chap. 6.

[5] For my views on Kant’s philosophy of biology in the OP, which was the main focus of my research on the OP, see van den Berg (2014), Chaps. 6–8. For more general discussion on Kant’s views on biology, see van den Berg (2013a) and (2013b).


Adickes, E. (1920), Kants Opus postumum dargestellt und beurteilt (Berlin: Reuther & Reichard).

Basile, G.P. (2013), Kants Opus postumum und seine Rezeption (Berlin: de Gruyter).

Beiser, F. (2002), German Idealism. The Struggle against Subjectivism (Cambridge, MA: Harvard University Press).

Berg, H. van den (2013a), ‘The Wolffian Roots of Kant’s Teleology’, Studies in History and Philosophy of Biological and Biomedical Sciences 44: 724–34.

——— (2013b), ‘Wolff and Kant on Scientific Demonstration and Mechanical Explanation’, Archiv für Geschichte der Philosophie 95: 178–205.

——— (2014), Kant on Proper Science: Biology in the Critical Philosophy and the Opus postumum (Dordrecht/Heidelberg: Springer).

——— (2016), Review of Bryan Hall, The Post-Critical Kant: Understanding the Critical Philosophy through the Opus postumumKantian Review 21: 329–33.

Eberhard, J.P. (1774), Erste Gründe der Naturlehre, 4th ed. (Halle: Renger).

Edwards, J. (2000), Substance, Force, and the Possibility of Knowledge: On Kant’s Philosophy of Material Nature (Berkeley: California University Press).

Emundts, D. (2004), Kants Übergangskonzeption im Opus Postumum (Berlin and New York: de Gruyter)

Falkenburg, B. (2000), Kants Kosmologie: Die wissenschaftliche Revolution der Naturphilosophie im 18. Jahrhundert (Frankfurt a/M: Klostermann).

Förster, E. (2000), Kant’s Final Synthesis: An Essay on the Opus postumum (Cambridge, MA: Harvard University Press).

Friedman, M. (1992), Kant and the Exact Sciences (Cambridge, MA: Harvard University Press).

Gloy, K. (1976), Die Kantische Theorie der Naturwissenschaft. Eine Strukturanalyse ihrer Möglichkeiten, ihres Umfangs und ihrer Grenzen (Berlin: de Gruyter).

Hoppe, H. (1969), Kants Theorie der Physik: Eine Unteruchung über das Opus postumum von Kant (Frankfurt a/M: Klostermann).

Mathieu, V. (1989), Kants Opus postumum, ed. Gerd Held (Frankfurt a/M: Klostermann).

Onnasch, E.-O. (ed) (2009), Kants Philosophie der Natur. Ihre Entwicklung im Opus postumum und ihre Wirkung (Berlin: de Gruyter).

Pollok, K. (2001), Kants Metaphysische Anfangsgründe der Naturwissenschaft: Ein kritischer Kommentar (Hamburg: Felix Meiner).

Skinner, Q. (1969), ‘Meaning and Understanding in the History of Ideas’, History and Theory 8: 3–53.

Tuschling, B (1971), Metaphysische und Transzendentale Dynamik im Kants Opus postumum (Berlin: de Gruyter).

Westphal, K. (2004), Kant’s Transcendental Proof of Realism (Cambridge: Cambridge University Press).

© Hein van den Berg, 2016.

Hein van den Berg is Assistant Professor of Philosophy at the University of Amsterdam and at the VU University. Van den Berg specialises in the history of philosophy, in particular the development of German philosophy and logic from Wolff to Kant, and in the history of biology, in particular eighteenth-century German physiology and embryology. Among publications in Archiv für Geschichte der PhilosophieBritish Journal for the History of PhilosophyStudies in History and Philosophy of Biological and Biomedical Sciences and Synthese, he is the author of Kant on Proper Science. Biology in the Critical Philosophy and the Opus postumum.