Some years ago, during my doctoral studies, my interest turned towards what Kant had to say about modality. I looked around, but at that time there was precious little written specifically on Kant and modality. This is the book I was looking for. It fills a yawning gap in Kant scholarship. This is not a gap of parochial or niche interest. Once one starts to learn more about Kant’s views on modality, one recognizes that modal concerns lie at the heart of a proper understanding of his philosophy. I happen to disagree with Stang in several areas about exactly how modality plays that central role, but I wholeheartedly agree that it does. Kant’s Modal Metaphysics presents a comprehensive account of Kant’s views – pre-Critical and Critical – about modality, but it also situates these views at the heart of Kant’s philosophy, and indeed at the heart of the Critical revolution. As such, this is an important book for anyone interested in developing a deeper understanding of Kant’s philosophy and philosophical development.

The book is organized into two parts: Part I deals with Kant’s pre-Critical, Part II with his Critical modal metaphysics. Part of the narrative offered by the book is that in his pre-Critical work Kant develops a number of positions about modality. In the development of his Critical philosophy, some are retained, but in addressing challenges and problems raised by his pre-Critical thinking, Kant leaves some of these positions behind. Hence, to properly understand Kant on modality, for Stang, we must look as much to his early work as to his later.

Part I introduces Kant’s foil: a ‘logicist’ metaphysics and epistemology. Stang’s choice of the label ‘logicist’ has the potential to be confusing, given its established use to refer to the early twentieth-century view that mathematics is reducible to logic: readers beware. According to (Stang’s) logicism, possibility simpliciter is logical possibility, and knowledge of possibility is accordingly logical and conceptual: something is possible in the widest sense if the concept of it is non-contradictory, and we can know that it is possible by inspecting our concept for contradiction (and not finding it there). One of the major theses put forward by Kant, and explored by Stang, is a rejection of this logicism. This also involves, for Stang, a rejection of ontotheism, the doctrine that God exists in virtue of his essence.

The book thus contains a detailed account of ontological arguments and potential rebuttals, which should be of interest to philosophers and theologians working in this area.

Once logicism is rebuffed, an alternative must be developed. Kant’s notion of real possibility is distinguished from mere logical possibility. For something to be really possible, it must satisfy further conditions than merely the concept or thought of it being free from contradiction. For Stang, it is distinctive of Kant’s real possibilities that they have actual, non-logical grounds. In the pre-Critical metaphysics, real modalities are grounded, ultimately, in God. Much of the development of these ideas, including an important attempt to argue for the existence of God qua absolutely necessary being that is the ground of all real possibility, occurs in Kant’s long essay ‘The Only Possible Argument in Support of a Demonstration of the Existence of God’ (the Beweisgrund) [Kant 1979]. Hence much of Part I is devoted to an examination of this text. Anyone looking for a detailed commentary on this text need look no further than Part I of Stang’s book.

Part II charts Kant’s Critical turn. The epistemological and semantical difficulties raised in Part I – how we can mentally represent and have knowledge of real possibility – are to be offered a solution by transcendental philosophy. Stang also presents a detailed account of the different kinds of modality defined in Kant’s Critical work. For Stang, this encompasses real possibility as a genus, of which formal, empirical-causal, noumenal-causal and nomic possibility are species. Drawing the parts together, in Chapter 9 Stang argues for ‘an unexpected high degree of historical and systematic unity to Kant’s modal metaphysics’ (p. 8). For example, according to Stang, the notion of grounding remains central to Kant’s modal metaphysics. Finally, Stang introduces and offers a solution to what he calls the ‘antinomy of Kant’s Critical modal metaphysics’, which concerns the kind of modal features that noumena can have, and how this impacts Kant’s account of freedom and the will.

The book is written in a clear and analytical style. This is in many ways a blessing, and allows one to engage straightforwardly with much of the material. However, one might have reservations about some of the content of contemporary analytic metaphysics that is introduced. For example, as emphasized, a central notion of Stang’s reading of Kant on modality is that of grounding. Apart from anything else, real possibility, arguably the central modal notion of the book, is defined in terms of grounding: ‘real possibilities have real grounds in actuality’ (p. 198). There is significant evidence that a notion of grounding is at work in the pre-Critical view. However, it seems strange to me that in the primary source for the Critical view, the section of the Critique of Pure Reason devoted to modality (the Postulates of Empirical Thinking in General, A218–35/B265–87),Footnote ² the notion of grounding hardly figures at all. The word, or variants, appears only twice, where Kant argues that the possibility of invented concepts is ‘groundless’, because they cannot be ‘grounded in experience and its known laws’ (A223/B270). It does not explicitly appear in the definitions of the modal concepts, nor their primary explanations. Whilst one can find passages connecting modality and grounds elsewhere in the Critical Kant, such as in his Lectures on Metaphysics,Footnote ³ it remains troubling that in the central text this aspect of the view, so important for Stang’s interpretation, is hard to find. This concern takes on a different appearance when viewed in light of the recent resurgence of interest in contemporary analytic metaphysics – unconnected with Kant scholarship – in the notion of grounding.Footnote ⁴ Did Kant really intend a mature account of modality in terms of grounding, or does Stang’s interpretation draw too heavily on contemporarily favoured philosophical methods and concepts in presenting Kant’s view in an analytical style? I am by no means asserting the latter, but readers should be alert to this kind of issue when reading the book, and take care to review the primary sources themselves accordingly.

Stang has certainly done more than anyone, in this book and elsewhere (see his 2011), to articulate in print Kant’s (Critical) account of different kinds of modality. This is undoubtedly a valuable contribution to Kant scholarship, and makes his work one of the first points of reference for any reader interested in Kant on modality. However, there is one aspect to his presentation of these different modalities that troubles me: the relationship between formal and real possibility.

The Postulates are introduced by Kant as concerning ‘things and their possibility, actuality, and necessity’, and not merely ‘the form of thinking’ (A219/B267). This accords with Kant’s rejection of logicism. Whether a thing is possible is more than a matter of the logical consistency of thoughts about it. This is the key idea of real possibility: really possible things are possible. Merely logically possible things are not.

In Chapter 7, Stang presents two conditions on a kind of possibility being real:

Kant’s first postulate presents a principle of possibility:

In Kant’s Critical philosophy, one of his key ideas is that there are conditions on possible experience. Objects of experience must be given to us: we must be presented with objects, we cannot simply create them with our minds. This is the material condition of experience. But the way our minds work also provides a form within with we experience these objects. We can only have experience of objects within the formal framework of space, time, causality, and so on. These are the formal conditions of experience. The principle of possibility is put in terms of formal conditions. Possible objects must conform to these formal conditions of experience; if they did not, we would not be able to experience them. Stang puts the same point in terms of different kinds of grounds for phenomena (objects of experience): immanent grounds – ‘grounds that are either themselves phenomena or are subjective conditions of phenomena, such as the forms of experience’ (p. 200) – and transcendent grounds – ‘grounds that are not immanent to experience, namely, noumena’ (p. 200). What I am calling formal conditions of experience are immanent grounds for phenomena – they are subjective conditions of phenomena.

From this first postulate, Stang extracts a notion of formal possibility.

In other words, the formal conditions of experience do not rule out p, where ‘rule out’ is to be understood in terms of grounding, not logical (in)compatibility.Footnote ⁵ Stang argues that formal possibility satisfies the conditions for real possibility, and hence is a kind of real possibility. It is non-logical: it is not a conceptual truth that if p is not self-contradictory, then it is not the case that facts about the actual forms of experience wholly ground the fact that ¬p; p might make a logically coherent claim about something that violates the forms of experience, such as an atemporal object. Formal possibility also satisfies groundedness: formal possibilities are defined as having a ground in actual forms of experience.

My worry is thus the following. I agree that formal possibility is a necessary condition for something to be possible. But it remains open whether it is also sufficient. If it is not, and formal possibility is a kind of real possibility, then something could be really possible (formally) without being a possible thing. This goes against the idea that real possibility is supposed to concern the possibility of things, and hence damages the idea of real possibility as an antidote to logicism. The problem arises because there are material conditions as well as formal conditions of possibility. It is not sufficient for something to be possible that it conforms to formal conditions, it must also be possible for such a thing to be given to us. Perhaps satisfaction of formal conditions guarantees satisfaction of the material condition, in which case formal possibility would be sufficient for possibility of a thing. But this requires further argument. In Stang’s terms, phenomena have both immanent and transcendent grounds. Formal possibility is grounded only by immanent, not transcendent grounds. Therefore, it is unclear whether something that is formally possible also has sufficient transcendent grounds or not.

The kind of possibility associated with what I am calling the material condition, and what Stang calls transcendental grounds, is what Stang himself calls ‘noumenal-causal possibility’:

The problem case would be some p that satisfied (i) – thereby being formally, hence really, possible – but not (ii). If no noumena could underlie the appearance of p, then no matter if p is formally possible, there could be no such truth about phenomena.

Without pursuing this problem further, we can see that there is at least work to be done within Stang’s interpretative framework. One must address the question whether formal possibility ensures the possibility of things, and if not, whether it is rightly called ‘real possibility’. To be fair, this is not just a problem for Stang: it is a (faithfully reproduced) problem that arises in Kant’s text. Kant defines possibility in terms only of formal conditions of experience, whilst also asserting that there are material conditions.Footnote ⁶ It would be interesting to see if, within this framework, Stang can answer the question why, in this context, Kant felt that only formal conditions were relevant to possibility.Footnote ⁷

In sum, Kant’s Modal Metaphysics should be a welcome addition to the bookshelf of anyone interested in modal metaphysics or Kant’s philosophy; it is an essential addition to the bookshelf of someone interested in both.Footnote ⁸