Moses Mendelssohn begins one of his last works, the 1785 Morning Hours: Lectures on the Existence of God, as follows:

Despite professing to be unfamiliar with recent intellectual trends, Mendelssohn expresses worries about a number of developments, including the threat posed to his own commitments by the ‘all-crushing Kant’, and a rising empiricism elevating ‘the senses’ over ‘speculation’.

While hardly universal, this image of Mendelssohn as superseded by Kant, and as committed to speculative rationalism but hostile to empiricism, has proven remarkably influential. It is this image, however, that Paul Guyer challenges in his impressive new book. Offering a wide-ranging comparison of these two figures, Guyer highlights not only their significant disagreements but also their striking affinities, and he draws on this analysis to illuminate the strengths and weaknesses of their philosophies. He advances two central claims, focusing on these thinkers’ engagement with rationalism and empiricism – with outlooks that he describes as emphasizing, respectively, concepts of objects originating in the intellect, and experiences of objects grounded in the senses. First, Mendelssohn and Kant inherit a shared rationalist tradition but defend its central tenets in different ways, with Mendelssohn offering theoretical arguments for God’s existence and the soul’s immortality, and Kant appealing to practical reason to ground these doctrines. Second, both philosophers seek to synthesize rationalism with empiricism, holding that – with several exceptions – knowledge requires both concepts originating in reason and the experiences of the senses. Indeed, despite his alleged hostility to empiricism, ‘in many ways Mendelssohn was the better empiricist as regards the concrete circumstances of human experience’, whereas ‘Kant had more of enduring value to say about the rational structure’ of knowledge and belief (p. 26). For Guyer, while Kant often deploys a richer account of reason to illuminate the categories, principles and ideas of human thought, Mendelssohn frequently provides a richer account of human experience in all its complexity.

After devoting chapter 1 to the rationalist and empiricist dimensions of Mendelssohn’s and Kant’s 1763 prize essays, Guyer takes up questions of metaphysics and epistemology, beginning in chapters 2–3 with the existence of God. He suggests that while these figures’ early treatments of this topic incorporate both rationalist and empiricist elements (albeit with important differences), their later writings diverge more strongly. Whereas Mendelssohn continues to refine and defend ontological and cosmological arguments for God’s existence (and develops a new demonstration focused on the incompleteness of human thought), Kant repudiates theoretical proofs in favour of treating this doctrine as a postulate of practical reason – in favour of arguing for the rationality of belief in God as one of ‘the necessary conditions of the possibility of our compliance with the demands of morality’ (p. 77). Chapter 4 considers these thinkers’ defences of immortality, showing that, despite appropriating elements of Mendelssohn’s position, Kant replaces his predecessor’s theoretical rationalism with arguments focused on practical grounds. Indeed, personal immortality plays a diminished role in some of Kant’s later works, perhaps because his conception of freedom undercuts the need for this doctrine.

Chapter 5 concludes the book’s first part with the topic of idealism. According to Guyer, Mendelssohn and Kant agree that we can affirm the existence of external objects even as we know nothing of their nature beyond how we represent them. However, while Mendelssohn is satisfied with a probabilistic defence of external objects’ reality rooted in the reliability of sensory experience, Kant argues for such objects’ existence on transcendental grounds as a condition of the possibility of self-knowledge. Similarly, Mendelssohn contents himself with the truism that we can represent objects only as we represent them (and thus cannot know whether they actually possess the spatial or other properties that they present to us), whereas Kant goes further and argues, again on transcendental grounds, that things in themselves cannot be spatial and temporal.Footnote ¹ Mendelssohn thus exhibits ‘a kind of epistemic modesty’ and ‘might be considered to be the purer empiricist’ (p. 168).

Chapters 6–8 focus on aesthetics. Mendelssohn is often remembered as the heir of a rationalist tradition associated with Leibniz, Christian Wolff and Alexander Gottlieb Baumgarten. For Guyer, however, Mendelssohn combines ‘his rationalist theory with a healthy empiricism toward the variety of actual aesthetic experience’ (p. 204), and in fact does so more consistently and thoroughly than Kant (despite some similarities). Kant typically links beauty to the formal, rather than material, features of artistic works – to the lines in a painting rather than its colours, or to a musical composition’s notes rather than the tones of its instruments. By contrast, Mendelssohn takes all such features to contribute to experiences of beauty. Moreover, Kant tends to ignore or minimize the role of emotions in our responses to art, whereas Mendelssohn ascribes considerable importance to the aesthetic expression and communication of sentiments and passions. One factor is Kant’s desire for consensus in aesthetic judgements, but a more fundamental issue is his commitment to grounding morality in reason rather than emotion, which leads him to downplay the sentiments’ role in morally significant aesthetic experiences.

The book’s final part focuses on religion, politics and history, beginning in chapter 9 with the concept of ‘enlightenment’. Guyer shows that Mendelssohn understands enlightenment in theoretical terms as the acquisition of knowledge regarding human perfection, whereas Kant frames enlightenment as the exercise of freedom and thus as a matter of practical reason. Chapters 10–11 turn to religious freedom and pluralism, suggesting that Kant provides a more compelling argument for the impermissibility of religious coercion. However, Guyer continues, Kant adopts a more problematic posture toward the actual existence of historical religions, unfairly criticizing the Jewish tradition that mattered so much to Mendelssohn, and implausibly holding that all historical religions will be superseded by a pure religion of reason. By contrast, Mendelssohn’s empiricism yields the more realistic view that religious diversity is unlikely to disappear. From his perspective, experience shows that the differences between groups living in different places and eras are sufficiently great, and the development of these groups sufficiently varied, to undermine the possibility of there being one universally effective mode of presenting religious ideas.

Chapter 12 concludes with Kant’s critique of what he calls Mendelssohn’s ‘Abderitism’ – Mendelssohn’s insistence that it is unrealistic to expect continuous moral progress for humanity as a whole. Guyer reads Mendelssohn’s position as another example of his empiricism. On this interpretation, Mendelssohn holds that, since the moral development of societies is dependent on the moral development of their individual members, and since individuals’ moral trajectories vary and can be determined only by experience, we cannot conclude that societies, and by extension humanity as a whole, will inevitably progress over time. Indeed, Kant’s optimism regarding progress has more in common with Mendelssohnian abderitism than he acknowledges, and his conception of human freedom itself undermines his critique of Mendelssohn. ‘Here’, we read, ‘is another point at which Mendelssohn’s common sense seems more plausible’ (p. 258).

This is an excellent, erudite book. Guyer’s arguments are compelling, marshalling a striking amount of textual and philosophical evidence that should put to rest the picture of Mendelssohn as an ‘unreconstructed pure rationalist’ whose ‘doomed philosophy … was destined to be destroyed once and for all by … Kant’ (pp. 2–4). Guyer judiciously highlights the strengths and weaknesses of each thinker, attending to the details of their intellectual development without losing sight of the broader philosophical issues at stake in their respective oeuvres. He is particularly adept at exploring cases where one thinker explicitly engages the other, along with instances where Kant might be implicitly appropriating and criticizing Mendelssohn. Indeed, part of what is impressive is the sheer number of such cases Guyer uncovers, from discussions of immortality and idealism in the first and second Critiques to the account of Christianity in Religion within the Boundaries of Mere Reason.

If I have a question about Guyer’s study, it is whether he sometimes overstates his case. Consider his discussion of moral progress. Focusing on a widely cited passage in the 1783 treatise Jerusalem, Guyer claims that Mendelssohn ‘applies an empirical rather than a priori epistemology’ (p. 325), and that what this empiricist approach yields is not a rejection of the very possibility of moral progress, but the more measured claim that we cannot treat it as inevitable. According to Guyer, while for Mendelssohn ‘experience does not prove the moral progress of humankind’ (p. 326), ‘Mendelssohn never denied that the moral progress of mankind or its political progress is possible’ (p. 332) and ‘did not attempt any a priori proof that progress is impossible’ (p. 335), and ‘nothing in Mendelssohn’s argument proves or seems intended to prove that moral progress is impossible’ (p. 336).

This may be a plausible reading of the relevant Jerusalem passage, and it supplies Guyer with another instance of compelling Mendelssohnian empiricism. Yet I am not sure that this interpretation accords either with Guyer’s broader argument about Mendelssohn as a synthesizer of rationalism and empiricism, or with the views Mendelssohn himself expresses elsewhere. Right around the time that he begins composing Jerusalem, Mendelssohn reflects in a 1782 letter on the idea that humanity is mired in a ‘cycle’ of progress and decline:

This is not the modest argument reconstructed by Guyer, but rather a more robust claim that moral progress is actually impossible – that ‘the human race must here and there go backwards if individuals are to advance’, and that because ‘each individual human being ought to develop his aptitudes and capacities and thereby ceaselessly become more perfect … the entire race must ceaselessly repeat [a] cycle’. Moreover, as these lines show, Mendelssohn grounds this conclusion in the idea, elsewhere established on a priori grounds,Footnote ² that the human being’s vocation or destiny is to pursue perfection by developing her faculties. If humanity were to undergo continuous progress in areas such as morality, human beings would no longer face the obstacles that spur them to the sorts of actions that develop their faculties and allow them to pursue their vocation; nature must be arranged in a way, then, that limits humanity’s progress, generating periods of regression that create hardships conducive to the striving necessary for the faculties’ development. Whether or not this is a compelling argument, it is one that Mendelssohn would repeat elsewhere during the same years, including in the short essay ‘On the Best Constitution’ (Reference Mendelssohn1971–: vol. 6.1, 145–6). Even if we accept Guyer’s empiricist reading of Jerusalem, then, it seems to me that he goes too far when he concludes that ‘Mendelssohn never denied that … progress is possible’ and ‘did not attempt any a priori proof that progress is impossible’. Rather, as with so many other dimensions of his thought, Mendelssohn blends empiricism with rationalism, developing both a modest argument (about the non-inevitability of progress) rooted in experience, and a more ambitious claim (about the impossibility of progress) grounded in an a priori account of the human vocation.

There may be other instances where we can raise similar questions. For example, Guyer is surely correct that Kant invokes practical reason to ground doctrines for which Mendelssohn offers theoretical arguments, and that an early case where Mendelssohn invokes ‘practical’ conviction regarding God’s existence, while ‘in the same general family’ as Kant’s position, differs significantly from the view that Kant eventually develops (p. 54; see also pp. 45–6). Nevertheless, Guyer passes very quickly over what strikes me as a more significant moment in which Mendelssohn might be read as moving in a quasi-Kantian direction. Just as Kant argues for God and immortality as ‘necessary conditions of the rationality of striving to fulfil the demands of morality’ (p. 269), Mendelssohn’s 1767 Phaedo defends belief in an afterlife by arguing that it is rational to fulfil our independently established moral obligation to sacrifice our lives for the state or other just causes (when necessary) only if we posit the soul’s immortality – indeed, that ‘without being convinced of … immortality’, it is only ‘the heart, and not reason’, that could yield compliance with this duty (Mendelssohn Reference Mendelssohn2007: 136–8, 158; Reference Mendelssohn1971–: vol. 3.1, 116–18, 156–7). Guyer discusses the broader section of the Phaedo in which this argument appears (along with that section’s potential links to Kant), but he says little about Mendelssohn’s reasoning regarding immortality and the rationality of moral action, simply quoting one line from this argument (without explication) and characterizing it as a ‘rather subsidiary’ dimension of Mendelssohn’s position (p. 154). Yet this argument is one that Mendelssohn singles out as meriting readers’ attention (Reference Mendelssohn2007: 43; Reference Mendelssohn1971–: vol. 3.1, 9), forcefully defends in the face of criticism (Reference Mendelssohn2007: 158; Reference Mendelssohn1971–: vol. 3.1, 156–7), and may refine and expand in his 1770 Hebrew commentary on the biblical book of Ecclesiastes (Reference Mendelssohn2018: 129–30; Reference Mendelssohn1971–: vol. 14, 152). Differences remain with Kant’s moral theology, but I wonder whether there are grounds to qualify Guyer’s contrast between Mendelssohn’s theoretical and Kant’s practical versions of rationalism.

In the end, though, none of these questions undermines the success of Guyer’s broader project. The points I raise, in fact, might be seen as strengthening his overarching argument, providing additional evidence for his picture of Mendelssohn as a philosopher who strives to reconcile rationalism with empiricism, and who has much in common with his ‘all-crushing’ foe. Guyer’s philosophically, exegetically and historically rich book is indispensable reading for anyone interested in Kant, Mendelssohn or eighteenth-century philosophy.