The chapter in the Critique of Pure Reason that is usually referred to as the ‘Metaphysical Deduction’ has long been the subject of discussion in the context of book-length interpretations of the Transcendental Deduction as it is the necessary preliminary to that chapter. It formed an important part, for instance, of Longuenesse’s masterful Kant and the Capacity to Judge (Longuenesse Reference Longuenesse1998). But there are not many self-standing book-length treatments of the Metaphysical Deduction alone, separately from the Transcendental Deduction. The classic monographs on the Metaphysical Deduction as such are Reich Reference Reich1932 (reprinted various times, and translated into English in 1992), Brandt Reference Brandt1991 and Wolff Reference Wolff1995. A fourth major monograph can now be added to this list: Till Hoeppner’s Urteil und Anschauung. Kants metaphysische Deduktion der Kategorien. Crucially, unlike those aforementioned three books, but like Longuenesse, Hoeppner includes an extensive, separate account of the all-important third section of the Metaphysical Deduction, that is, the section leading up to the table of categories (§10, A76–80/B102–6).

Hoeppner does not spend time on the question whether Kant’s arguments make any sense in the context of developments in logic after Kant. He specifically attempts to provide a precise interpretative reconstruction of the arguments so as to present them as coherently and persuasively as possible given Kant’s premises. Hoeppner also thinks that the argument of the Metaphysical Deduction, if we read it in the way he does, can be considered successful, including its ill-famed ‘completeness’ claim. Wolff’s book, and in its own superbly obscure way, Reich’s too, of course also attempted to present the Metaphysical Deduction in a positive light but, unlike many contemporary Kantians, it is refreshing to see Hoeppner painstakingly and rigorously reconstruct Kant’s own arguments, over close to 400 pages, without impatiently framing it in the terms of contemporary philosophical concerns.

It is noteworthy that Hoeppner is not so much interested in issues of a more philological concern, such as the question why the chapter in the Critique that is at issue is standardly called ‘Metaphysical Deduction’ in the first place, or, more precisely, why Kant casually gives it that label only much later, toward the end of the Transcendental Deduction. Nowhere in the actual chapter, which runs from A66/B91 to A80/B106, does the name in fact appear. None of the chapter headings of the three main sections that make up the actual deduction (§§8–10) contain the words ‘Metaphysical Deduction’. Instead, Kant consistently speaks, in all three headings, of a ‘guiding thread’ to the ‘discovery’ of the concepts of the understanding, or also, in the heading of the third section, the categories. The very term ‘deduction’ only first appears at A84/B116, at the start of the second chapter of the Analytic of Concepts, which is where the transcendental deduction of the categories begins, or at least the first of the two introductory sections that lead up to the Transcendental Deduction proper. This is of significance, I think, since it could be argued that what we usually call the ‘Metaphysical Deduction’ is not actually a deduction in the narrow sense of the term if we take seriously Kant’s account in §13, where Kant conceives of the Deduction analogously with how jurists required proofs of both the quaestio facti and the quaestio juris. Only the quaestio juris would strictly speaking, that is, legally speaking, be a deduction, whereas the quaestio facti, which on such an account would be equally as important, is not.Footnote ¹ This goes against the grain of the received interpretation, since most scholars, the present reviewer included, think that the quaestio facti is irrelevant to Kant’s argument, or merely a foil for it (with Proops Reference Proops2003 being an important exception). I am not sure if that argument is right, but if indeed what usually is called the ‘Metaphysical Deduction’ is not a deduction in the strict sense of the term but concerns the quaestio facti as integral part of a larger argument for the objective validity of the categories of which the Deduction strictly speaking is also part, then this has repercussions for accounts of the argument in the so-called ‘Metaphysical Deduction’.

For one thing, though it is entirely legitimate to delimit one’s object of study for the purposes of a book, on that account it would be difficult to maintain that the Metaphysical Deduction is a perfectly self-contained set of arguments. And indeed, while he does not go as far as Reich who for the most part of the evidence for his reading looked outside the actual sections that make up the Metaphysical Deduction, or in fact frequently outside of the Critique altogether in Reflection notes, Hoeppner seeks support for one of his main claims about what constitutes the categories in the subjective deduction in the A-edition of the Transcendental Deduction, and for support of his, in my view, questionable reading of the famous passage at A79/B104–5 (about which more below) he looks to a section in the Transcendental Dialectic as his best evidence. I, for one, am not against reading the claims in the Metaphysical Deduction in the light of later treatments in the Transcendental Deduction or indeed by resorting to other sections in the Critique or to Reflection notes. But this goes to show that it is hard to present a self-contained reading of the Metaphysical Deduction and not to run ahead of the argument for, e.g., ‘synthesis’ in §10, as a crucial component of the derivation of the categories from the functions of judgment, by looking at the arguments for synthesis in the Transcendental Deduction for major interpretative support, as does Hoeppner.

Nevertheless, the interpretative support, internal and external to the actual text of the Metaphysical Deduction, that Hoeppner does muster in support of his reading is impressive. His analysis of Kant’s arguments for the central claim that the elements that make up the conditions of thought are exactly congruent to the functions of judgment, as well as his intriguing and, at first pass, persuasive account of the categories as having the threefold act of synthesis as their representational content (330 et passim) are rigorously argued, exceedingly detailed and frequently illuminating. I am also in complete agreement with Hoeppner that the whole of the Metaphysical Deduction is part of transcendental logic (27, 60n.143, 331), so from the start of the chapter at A67, whereas it is often thought, mistakenly, that the arguments especially in the first two sections, which concern the elements of thought and the table of the functions of judgment, are part of general logic. He is also right of course that the relative tasks of the Metaphysical Deduction and the Transcendental Deduction can be fairly easily distinguished, respectively, as an argument that shows what the necessary conditions of thinking and thus judging are and an argument that shows that these conditions are sufficient for certain concepts also to be objectively valid (23). But I was not always sure Hoeppner was consistent in differentiating these tasks, while he often talks about objective validity and ‘objectiven, wahrheitsfähigen Urteile’ (64, 178 et passim) in the context of the Metaphysical Deduction, an ambiguous notion he does not define nor really explain, but just assumes. It is true that Kant later, in the Transcendental Deduction, defines a judgment as an objectively valid unity of representations (B141–2), but this definition cannot already be assumed without further argument in an account of §§8 and 9 of the Metaphysical Deduction.

In a short review of such a monumental book on such a complex theme, I cannot possibly detail every theme or aspect that Hoeppner discusses nor flag all the potential problems and apparent contradictions that I encountered. But the one major issue I have with his take on the Metaphysical Deduction, in particular his reading of §10, cannot remain unmentioned. Based on his distinction between the ‘logical’ and ‘real uses’ of the understanding – the main textual evidence for this he gathers from A299 – he sharply distinguishes between analysis and synthesis as irreducible acts of the understanding. This ties in with his view that the acts of synthesis of the manifold in an intuition are not judgments, even though they are acts of the understanding (79, 119, 210, 353). First, this would appear to conflict with Kant’s explicit claim that ‘we can trace [zurückführen] all actions of the understanding back to judgments, so that the understanding in general can be represented as a faculty for judging’ (A69/B94); this was why the functions of judgment could be derived so neatly from the understanding as the capacity to think in the table of judgment in §9. Of course, acts of synthesis are not themselves judgments, but this does not imply, as Hoeppner seems to believe, that acts of synthesis can take place outside or independently of judgments; these acts being conditions of judgment (295) – Hoeppner speaks somewhat incongruously of them as the Seinsgrund of judgments that in turn are themselves the Erkenntnisgrund of these acts (325) – they are an inseparable part of judgments nonetheless.

Secondly, and perhaps even more importantly, this sharp distinction between analysis and synthesis threatens to undermine what Hoeppner himself calls the ‘Identitätsthese’ (348ff.) that Kant puts forward in the famous Leitfaden passage at A79/B104–5. Why is that? Hoeppner reads the identity claim that is expressed by Kant’s phrases ‘dieselbe Funktion’, ‘derselbe Verstand’ and ‘dieselben Handlungen’ non-reductively, namely, as not amounting to the claim that one of each act is to be seen as the other and can be reduced to it (353). Hoeppner argues that the identity that is meant here is a ‘generic’ identity, namely, both acts, the act that is responsible for the form and unity of a judgment (analytic unity) and the act that is responsible for the unity of an intuition (synthetic unity), are different species of the same genus, whereby genus is the understanding in a higher form; they are not identical in the sense that they would literally be the same act. Hoeppner translates Kant’s phrasing, somewhat awkwardly, by speaking of ‘generisch dieselbe Funktion bzw. generisch dieselben Akte generisch desselben Verstandes’ (354). The problem with this, it seems to me, is twofold.

First, while Hoeppner focuses on the fact that the understanding has two different uses or applications (‘logical’ and ‘real’), on the level of the form of judgment and on the level of the representational content in intuition, which justifies the distinction between the means of analytic unity and the means of synthetic unity, respectively, he skirts around the real question about the identity claim in the passage, namely to specify what ‘dieselbe Funktion’ or ‘dieselbe Handlungen’ are that have these distinct applications or uses. Just saying that there is such a higher genus of which the distinct means (analysis and synthesis) are species, namely a higher, generic concept of the understanding that contains what is common to the logical and transcendental concept of the understanding (355), is not the answer to the question what that same function is, which manages to achieve two distinct aims and yield two distinct results. It is just a shifting of the question. Longuenesse and others claim that this identical function is the logical function of judging, given that the understanding is basically the capacity to judge. But Hoeppner explicitly rejects this reading (371).

Secondly, given that Hoeppner supports the claim that the categories, which have the acts of synthesis as their representational content, are exactly correspondent to – he uses the term ‘entsprechen’ (286) – the functions of judgment (341–2), and given that he argues that the logical use of the understanding requires the real use of the understanding (179, 322) and the latter is in fact the ‘Seinsgrund’ of the former, then how is it that the acts of synthesis that constitute the categories, as Hoeppner himself argues quite rightly, are not to be seen as that which also constitutes the forms of unity in judgments? Hoeppner of course believes that the categories, and hence the acts of synthesis that form the original content of the categories, also have their origin in the understanding, that is, in the same understanding in virtue of which we judge. But the strong sense of correspondence between the act of synthesizing and the act of judging is lost when Hoeppner cannot explain by which ‘same’ acts, on Kant’s own account in A76/B104, the understanding achieves this dual performance other than by saying it is the same understanding that does it in two different modes. By denying that it is the act of judging, he seems to believe that a judgment is just the formation of the forms of unity on the conceptual level by means of an analytic unity, and not also and at the same time, in virtue of the categories, a determination of the manifold in an intuition, which would appear to contradict his other view that the logical use of the understanding does require its real use, in that manifolds with ‘real’ representational content need to be determined for it. It seems that synthesis and the act of judging are kept strictly separate in Hoeppner’s account; judgment and synthesis of intuition are merely ‘generically identical’ (360). But it is difficult to see how this comports with Kant’s clear belief that the ‘categories are nothing other than these very functions for judging, insofar as the manifold of a given intuition is determined with regard to them’ (B143). It seems to me that Hoeppner fails to grasp the real strong identity claim that lies in Kant’s Leitfaden passage at A76/B104–5, which says that our capacity to judge really is our capacity to determine objects that we intuit for our judging.

From a significant note toward the end of his account, in which he criticizes James Conant on this point (363n.660), it becomes clear that Hoeppner believes that, quoting Conant (Reference Conant2016), the ‘generic form of unity’ that judgment and synthesis of intuition have in common cannot be the ‘original synthetic unity of the understanding’ or ‘the original synthetic unity of apperception’, as Conant believes, as this would be tantamount, in Hoeppner’s view, to the reductive reading of the Leitfaden passage, for apparently it reduces analytic unity to a synthetic unity. I think Hoeppner here mistakes the synthesis in intuition for the ground of this synthesis, namely the original synthetic unity of apperception, which is in fact the spontaneity of the understanding (B130). I submit it is this original act of synthesis that is ‘dieselbe Funktion’ that unites, ‘by the very same actions’, the representations in a judgment, its logical form, and the representations in an intuition, a judgment’s transcendental content. This is not a reductive reading, it seems to me, given that Kant himself makes it quite clear that analytic unities of representations too require this original synthetic unity of apperception as their transcendental-logical ground of possibility (B130, 132–5).

Much more of course can be said about Hoeppner’s excellent book, and I am certain many scholars of Kant will benefit from its extensive and detailed analyses of an often still largely misunderstood chapter in Kant’s Critique of Pure Reason.