The main thesis of Kant on Laws is that Kant has a ‘generic’ notion of law which applies uniformly across his practical and theoretical philosophy. Watkins explains in chapter 1 of Part I that Kant’s view affords an understanding of laws of nature, as they figure in early modern science, within the context of the natural law tradition in practical philosophy. On this tradition, laws owe their ‘binding force’ to the authority of the legislator who prescribes them, and are binding only for rational beings aware of their obligatory force and free to disobey them. Watkins maintains that, for Kant, laws owe their ‘lawfulness’ (binding force) to the authority of reason, which legislates both the moral law and laws of nature. Being enacted or instituted by a suitable authority is one of two elements of the generic notion of law; the other is necessity, which Watkins understands as ‘relatively abstract’ and ‘thus tak[ing] on more specific forms in the case of the different kinds of laws’ (p. 27). The notion of necessity involved in the moral law is that of obligation, which does require ‘the rationality and freedom of whoever is obligated’ (p. 27), while the notion of necessity involved in laws of nature is determination, which does not.

The second of Part I’s two chapters focuses on the case of causal determination (which paradigmatically illustrates, but does not exhaust, the necessity belonging to theoretical principles; see p. 23). In this case, Watkins maintains, necessity ‘is not an inexplicable free-floating modality, but rather is based on the natures of things that substances act in accordance with in exercising their causal powers’ (p. 34). Anticipating the worry that these natures are discoverable only by ‘standard empirical inquiry’, which would rob Kant’s account of its ‘specifically transcendental dimension’ (p. 35), Watkins allows that these natures can be ‘subjective in character’ and ‘identified on the basis of being required for us’ to have some ‘basic kind’ of experience, for example, as spatiotemporal and in accordance with the categories (p. 36).

In Part II of the book, which is the longest by far and ranges the furthest chronologically over Kant’s own writings, Watkins extends this account of ‘transcendental’ laws of nature to the three ‘Laws of Mechanics’ in Kant’s Metaphysical Foundations of Natural Science (MFNS). Watkins argues that these Laws are transcendental because they are necessary for ‘experience of … the communication of motion’, which he counts as ‘experience of a very general sort’ on the ground that ‘every spatial object we could encounter is capable of communicating motion to other spatial objects’ (p. 35). A highlight of the book is Watkins’ thorough consideration of the historical context of Kant’s Laws, which both promises to explain Kant’s distinctive formulation of them and supports Watkins’ contention that Kant’s transcendental account of knowledge has implications for ontology as well as epistemology (p. 116).

I think the synoptic treatment offered in Part I affords a great deal of insight. While I have no objections to Watkins’ overall account, I do have questions about how it applies to the laws of physics.

Kant says the method of MFNS is to carry the empirical concept of matter through the ‘functions of the concepts of the understanding’, in four chapters, each time adding one new predicate which ‘traces back’ to motion, the ‘fundamental determination’ (4: 476). The empirical character of the concept of matter and the contingencies involved in the experience of motion seem to make the principles Kant derives less inevitable than the first Critique’s principles of the understanding. In particular, I am not convinced that communication of motion is as general a feature of experience as Watkins claims. Watkins takes Kant to hold that ‘only the communication of motion could express matter’s relation to other matter insofar as motion is concerned’, so that even one part of matter’s imparting of motion to another is ‘an instance of the communication of motion’ (p. 82). But Kant sharply distinguishes between communicating and imparting motion, and seems to give conceptual priority to the latter: imparting results from the ‘original’ forces considered in the second chapter, while force for communicating motion, which matter has only insofar as it itself moves, is a further determination of matter introduced in the third chapter. It would thus seem that experience of imparting motion, without communicating it, is possible in some sense, even if not possible all-things-considered. In the second chapter, moreover, Kant makes explicit that his ‘dynamical’ concept of matter (on which matter fills space through the interplay of original forces, 4: 517) is an alternative to how matter is conceived in a ‘merely mathematical physics’, which latter is able to ‘present what we thought universally as possible in intuition’ (4: 525). So it is again unclear that experience of some general or basic kind is possible only on the principles Kant sets forth (in this case, the conception of matter as possessing original forces).

Part III of the book complements Part II by considering the teleological principles that Kant employs where he finds mechanical laws insufficient. Watkins offers a clear, probing examination of the conflict between regulative principles that drives the third Critique’s Antinomy of Teleological Judgment. Without claiming to definitively resolve the conflict, Watkins indicates it is to be resolved by taking account of the subordination of mechanical explanations to teleological ones and the dependence of both on the super-sensible. The importance of teleological laws is made further evident by Kant’s contention that not only organisms and their parts, but rather ‘nature as a whole’, must be regarded as having purpose. Watkins explains this widening of the domain of teleological explanation in terms of reason’s search for the totality of conditions for anything that is conditioned.

Part IV treats the a priori laws that Kant ‘calls regulative rather than constitutive’, focusing on principles that were ‘traditionally at home in rational cosmology’ (such as the laws of no fate, no leap and no chance) and the ‘logical laws’ of homogeneity, specificity and continuity (p. 189). Watkins’ discussion of the cosmological principles bears out his view, already argued for in Part II, that Kant’s acceptance of Newtonian physics did not lead him to wholly reject Leibnizian principles, and that his Critical turn does not amount to forsaking metaphysics.

I find some tension in Watkins’ account of regulative principles within the theoretical philosophy. Watkins acknowledges that the principles ‘have a status similar to that of the Categorical Imperative’ (rather than the laws of nature) insofar as the necessity belonging to them is ‘an obligation to act (broadly construed) rather than determination’ (p. 271). In particular, they ‘direct the understanding’s activities in judgment’, directing it to seek various kinds of unity (p. 217); but, as Watkins emphasizes here, they do not guarantee the existence of the entities thereby sought. The problem with this way of incorporating regulative principles into the account of laws in general is that it seems to allow no explanation of their metaphysical import (which Watkins also stresses, in arguing that the principles are not merely methodological). A further concern is that Watkins may underestimate the importance of reflective judgement for regulative principles. While Watkins acknowledges that Kant may have ‘changed his mind’ or ‘supplemented his view’ of its importance between the first and the third Critique, he nonetheless maintains that ‘the employment of reflective judgment does not preclude the involvement of reason’ (pp. 273–4; cf. 175, n. 2). But Kant writes in the First Introduction to the third Critique that ‘neither understanding nor reason’ can establish a priori such laws as homogeneity, specification and continuity, which rather express ‘the power of judgment in establishing a principle … for its own needs’ (20: 210).

Part V considers Kant’s notion of law in the context of his practical philosophy. Its first chapter conjectures that Kant arrived at the doctrine of autonomy in the Groundwork on the basis of parallels with the Prolegomena’s account of how the understanding prescribes laws to nature. The second chapter begins with the observation that the role Kant assigns to reason, in prescribing moral laws and laws of nature, had traditionally been reserved for God. Watkins contends that, ‘rather than simply eliminating the divine order’, Kant instead ‘reestablishes [the] priority relations’ between the natural, moral, human and divine orders’ (p. 226). Watkins argues, specifically, that Kant ‘begins with undeniable features of our experience of the natural and moral orders’ and shows that ‘they are possible only if supported by highly specific features of a divine order’ (p. 249). This further supports his contention that Kant does not reject metaphysics in his Critical period.

The Conclusion, together with the chapter on the natural, moral and divine orders, shows in gratifying detail how the book as a whole amounts to more than the sum of its parts. Here Watkins argues that, if we identify the ‘supersensible ground’ invoked to resolve the third Critique’s ‘Antinomy’ with the being that is shown in the first Critique to exist (as the ground of all possibility) and shown in the second Critique to have ‘perfect moral qualities’, then we can better understand Kant’s resolution (p. 249), and trace an argument for God’s existence through all three Critiques. We can, furthermore, understand the subordination of mechanical to teleological laws by considering how God ‘will make the highest good attainable through our action’ (p. 264). The book ends by specifying the contributions of the various kinds of laws discussed to the ‘complete systematic theoretical and practical cognition of the unconditioned’ sought by reason (p. 282).

Watkins explains in the Preface that ‘the vision that inspired this book slowly took concrete form’ as he ‘lightly revised’ his previously published articles on seemingly disparate topics. A final set of questions concerns how Watkins’ treatment relates to others that have appeared in the years (in some cases, decades) since the articles first appeared.

Watkins opposes his own account of the transcendental status of the Laws of Mechanics to what he calls the ‘standard view’ that they arise by ‘substitut[ing] the concept of matter into the first Critique’s Principles of Pure Understanding’ (p. 72), which he finds in two articles published in the mid-1980s. The appearance of Michael Friedman’s Kant’s Construction of Nature (2013) has changed the scholarly landscape, with the effect, I think, of moving the ‘standard’ view much closer to Watkins’. To be sure, Friedman finds more continuity than Watkins between Kant’s Laws of Mechanics and Newton’s Laws of Motion. Yet Friedman presents MFNS as an account of how the concepts involved in Newton’s theory of motion can have ‘quantitative structure’, which is not so far from Watkins’ view that it gives conditions on some basic and pervasive kinds of experience. By contrasting his view with the older one rather than Friedman’s, Watkins misses an opportunity to bring his own contribution into focus.

Although Watkins claims Kant’s Critical treatment of ‘rational cosmology’ has received little attention (p. 191), Alison Laywine has published a number of articles on relevant topics, culminating in her 2020 Kant’s Transcendental Deduction: A Cosmology of Experience. In particular, in (2003) Laywine thematizes an issue that Watkins raises in passing (pp. 24–5): why sensibility is not a source of laws, on a par with understanding. Again, drawing a contrast would serve to highlight Watkins’ distinctive contribution.

Important recent scholarship on Kant’s predecessors also prompts questions about some details of Watkins’ account. Watkins argues that the laws of motion stated in book one of Principia are ‘absolutely fundamental’ for Newton’s project of developing mathematical principles of natural philosophy. Hence, ‘the slightest of variations in their formulations could invalidate the derivations of the later Books’, and in particular that of universal gravitation. Accordingly, eighteenth-century philosophers who formulated the laws of motion differently can be assumed to have different ‘fundamental concerns’ (pp. 92–3). But George E. Smith’s research on Newton’s thought reveals that Newton himself entertained different versions of at least the Third Law (Smith Reference Smith2007: section 5), which calls this inference into question. And it may be worth reconsidering the arguments of MFNS in relation to Leibnizian-Wolffian thought, now that Watkins has so compellingly demonstrated its relevance to Kant’s natural philosophy, and a wider range of thinkers is studied. In particular, we might revisit Watkins’ claim that for Kant ‘whether bodies are absolutely hard is presumably a contingent matter’ (p. 137). While Wolff declines to pronounce on whether there are absolutely hard bodies in nature (Reference Wolff1731: §383), Leibniz’s argument that they are precluded by the continuity of change was endorsed by Euler (as Watkins observes, p. 106) and by Émilie Du Châtelet (Reference Du Châtelet1740: §15). Indeed, Kant appears to mount a version of it (4: 552).

Raising these questions about Kant on Laws should not detract from its impressive achievement. As an original and surpassingly clear treatment of a notion central to both Kant’s theoretical and his practical philosophy, it deserves the attention of every serious student and scholar of Kant.