1. From theoretical to practical reason

As already indicated, the main claim of AR is that Kant fails in his attempt to unify reason in the Architectonic section of the first Critique. This applies, according to Ypi, both to the theoretical and the practical routes. Having examined existing interpretations of the question concerning the unity of reason and defended the relevance of the Architectonic to answering it in the book’s first part, Ypi dedicates its second part to Kant’s attempt to justify systematic unity in the Appendix of the Transcendental Dialectic from the point of view of reason’s theoretical interest. To do so is vital in order to make good on the idea of a systematic conception of knowledge and the pertinent difference between a system and an aggregate of cognitions.

The primary culprit for the failure of this project turns out to be the unclear or indeed underdeveloped notion of purposiveness. According to Ypi, in the first Critique Kant endorses a notion of purposiveness as design, which anchors the relationship between reason and nature in the idea of God (AR, pp. 112–19). Hence, the price Kant pays for architectonic unity of the initially separate systems is a dogmatic metaphysics that endangers the entire critical enterprise. It is not until much later, in the third Critique, that he would be able to replace the notion of purposiveness as design with purposiveness as normativity. There, the faculty of judgement mediates between a way of conceiving objects as given in experience (hence, as determined by laws of nature and compatible with the laws of the understanding), on the one hand, and the possibility of conceiving them as determined by human freedom and the final end of reason, on the other. This eventually allows him to conceptualize systematic unity without slipping into physical theology.

As already mentioned, Kant’s attempt to justify systematic unity in the Appendix of the Transcendental Dialectic from the theoretical perspective is only one part of his project. In the third part of the book, on which I shall focus my attention, Ypi turns to Kant’s attempt in the Canon to justify systematic unity from the perspective of reason’s practical interest: a project that answers to the need to satisfy the destination of human reason from a moral perspective. Focusing specifically on the essential ends of practical reason and the conditions under which nature and history can be conceptualized in organic unity, Kant asks whether ‘from the point of view of its practical interest reason may not be able to guarantee that which in regard to its speculative interest it entirely refuses to us’ (CPR, A804/B832). Kant himself seems optimistic that pure reason in its practical use is indeed able to reflect on ideas of the unconditioned without stepping beyond its boundaries, for its principles are importantly different from those involved in its speculative use. Specifically, practical reason is supposed to possess a kind of causality that can be exercised regardless of all empirical conditions, thus legitimizing a use of ideas that is not confined to the purely regulative one.

Unfortunately, Ypi argues, Kant fails to make good on this claim. The fatal problem is that, in the first Critique, practical reason has no domain for its own legislation (which Kant would first conceptualize in the second Critique). What would need to be shown in order to successfully unify reason via the practical route is that pure reason in its practical use has its own causality that can be exercised regardless of all empirical conditions, i.e. that human actions are both rational and causally efficacious in a world determined by natural laws. While this would require a notion of transcendental freedom (as independence from all sensible causes), the Canon reduces freedom to practical freedom, understood here as free choice or arbitrium liberum (AR, pp. 141–2): the power we have to ‘overcome impressions on our sensory faculty of desire by representations of that which is useful or injurious even in a more remote way’ (A802/B830). This notion, however, cannot provide reason with an unconditionally valid moral end. Kant thus problematically asserts that practical freedom can be a cause without laying out how this can be the case. This leaves him unable to justify a kind of systematic unity that is independent from the systematic unity of nature yet does not transcend the realm of sensible causes.

The upshot is that the problematic notion of purposiveness catches up with Kant once again: to the extent that reason has a practical dimension in the first Critique, the unity of its laws relies on whatever theoretical account of reason we have been offered up to that point. The ensuing problem runs parallel to that of the speculative case: in linking the principle of conformity to ends with the idea of the whole, Kant uncritically identifies the moral destination of human beings with the final end of nature. Ultimately, he slips from the necessary postulate of a supreme being (as the foundation and cause of the systematic unity of nature) to affirming the existence of such a being. While the speculative use of reason leads into physical theology, the practical use of reason leads into moral theology (AR, pp. 165–6). For the sake of reason’s systematic unity, Kant is forced, again, to violate the critical commitments laid out in the earlier parts of the first Critique.

2. The needs of practical reason

I want to start by looking more closely at the question why practical reason has a need for systematic unity. Why are there theoretical conditions on practical activity? Put differently, why do we need to make certain assumptions about the world, specifically about the harmonious unity between practical ends and the laws of nature, in order to (be able to) act consistently? Helpfully, Ypi distinguishes three different answers to this question that have been variously ascribed to Kant (AR, pp. 161-2); three different reasons, that is to say, behind Kant’s argument for the necessity of systematic moral unity from the point of view of the practical use of ideas.

According to the first model, practical ideas are necessary for moral motivation. Notice that this would seems to be the most plausible view precisely if we take our cue from the first Critique, as Ypi does. For in the Canon (A812/B839) Kant, somewhat enigmatically, pictures God as using ‘promises and threats’ to get us to do the right thing. Practical ideas, foremost the notion of a wise author of the world, constitute ‘incentives for resolve and realization’ (A813/B841). Not implausibly, Ypi follows interpreters such as Henry Allison (Reference Allison2011), who have argued that this is a remnant of Kant’s pre-critical view that we need affective incentives to follow the moral law. Accordingly, she follows a second line of interpretation. On this view, belief conditions on agency are a matter of rational consistency. According to what Marcus Willaschek (Reference Willaschek, Reath and Timmerman2010) calls the ‘realizability principle’ – basically, a version of ‘ought implies can’ – an agent is rational in pursuing an end if and only if they believe that the end is attainable.

Notably, Ypi opts for this reading not (primarily) for exegetical but rather pragmatic reasons. She highlights that it has the ‘crucial benefit’ of allowing us to understand the need for systematic moral unity while ‘bracketing … the psychological and motivational objections that afflict Kant’s framework in the first Critique’ (AR, p. 161). This is curious, given that the larger strategy of AR is precisely not to bracket objections of this kind but to unpack them. Given that the motivational reading is defended by Kant precisely in the very section of the first Critique that is at the heart of Ypi’s concerns, it (and its implications) would at least seem to warrant closer scrutiny.

Now, there is also a third account that conceptualizes the need for practical ideas as a matter of psychological necessity. According to what Andrew Chignell (Reference Chignell, Barnhill, Budolfson and Doggett2018: 299), for instance, has labelled Kant’s ‘consequence-dependent moral psychology’, finite agents will be demoralized or despair in pursuing ends by the prolonged experience that their efforts make no difference. This is a line of argument that, according to its proponents, we find particularly in Kant’s later writings such as the Religion, where Kant argues that we need our ‘well-intentioned efforts’ (6: 101) at least occasionally to be fulfilled in order to avoid despair.

At times, Ypi suggests that this is what is actually going on in Kant’s argument. At one point, for instance, she argues that finite beings like us ‘cannot avoid being interested in the consequences of their actions, and in what they can hope for in association with their promotion of moral imperatives. A world in which the laws of nature obstruct the practical use of reason would render absurd all human attempts to promote moral ends in the sensible world’ (AR, p. 159). Two things stand out about this passage. First, the appeal to an interest in the consequences of our actions and the looming prospects of absurdity makes it sound like we are fending off a threat of despair rather than a threat of rational inconsistency. Second, it is not entirely clear why, from this moral-psychological consideration, a need for systematic unity would even follow. What Ypi here suggests, I take it, is that in order to fend off the threat of despair we need to fend off the impression that our efforts are in vain. What we need to be committed to, in other words, is that the world is open to our intervention. It is at least not obvious why such a concern for the efficacy of my actions would necessarily imply a concern for an ultimate end, as long as the relevant action can be described as contributing to some desirable state of affairs. To take an example from Kant’s politics, why would I have to hope for perpetual peace if my aim were only to make the world more just?

Regardless of which of these three readings we opt for, a second and related question comes up. What does practical reason demand, modally speaking? What, in other words, do we need to assume to be the case about the world in order to satisfy practical reason’s interest? Systematically, Ypi’s interpretative take entails a strong modal connection between the order of nature and the order of ends as actually reconciled. The two are linked, according to her model, in a purposive system that integrates the concepts of nature and freedom with reference to the idea of God.

This view, Ypi further points out, also commits us to a teleological reading of history. For the idea of purposiveness as design ‘makes history intelligible not as a blind unfolding of random facts, events, and characters but as a sequence of events which have a purpose – the purpose of facilitating the coordinated and continuous integration of moral ends’ (AR, p. 166). Indeed, Kant occasionally makes remarks along these lines in his writings on politics and history. Sometimes, nature or history are characterized as conducive to our efforts, for instance when he claims that signs of history (such as the French Revolution) ‘encourage the hope that after many revolutions … the highest purpose of nature, a universal cosmopolitan existence, will at last be realised’ (‘Ideas for a Universal History’, 8: 29). More radically and (in)famously yet, in Perpetual Peace (TPP, 8: 360) Kant even claims that nature ‘guarantees’ or ‘wills’ perpetual peace to come about irrespective of human efforts.

How to reconcile this appeal to nature with the argument from duty has been the subject of sustained dispute among interpreters (e.g. Kleingeld Reference Kleingeld1995: 62–7). Surely, if nature can and will achieve the pertinent task even against individuals’ wills, then whether agents acknowledge any duties on their part makes no difference. Conversely, if individuals do act on their obligations, what job is there left for nature to do? Even if we highlight that the ‘guarantee’ argument is formulated from a distinctly practical standpoint, it ends up obscuring the role of human agency. The supposed certainty about nature’s assistance is neither required for practical purposes nor is it warranted from an epistemic perspective. Rather than supporting our resolve, it is ultimately more likely to lead us to lean back and watch the ‘great artist nature’ do her work.

In fact, we have seen previously in this section that neither of the three accounts available (from motivation, rational consistency or moral psychology) requires us to assume that our ends are more than merely possible. Ypi herself remarks at one point that, according to Kant, ‘human beings must pose their moral ends in the phenomenal world assuming that at least the possibility of a harmonic unity between such ends and the laws of nature is given’ (AR, p. 43; my emphasis). It is a separate and indeed tricky question what it means to (be warranted to) assert the possibility of our ends. Below, I will suggest that the ‘postulates’ (God and immortality) provide the ground for our commitment (by way of hope) to the possibility of the Highest Good. However, Kant seems to shift his view in this regard. Particularly in his late writings on politics, the conditions of warranted assertability are significantly lower. There, all that matters is that history or nature cannot be shown to rule out our ends as impossible. In the Doctrine of Right, for instance, he argues that we are licensed to assert the attainability of a proposed end even amidst difficult odds, as long as its impossibility ‘cannot be demonstrated either’ (Metaphysics of Morals, 6: 354).

The larger point is that Kant seems to make the case for a weaker and metaphysically less contentious type of modal connection between theoretical and practical commitments than the one suggested by Ypi. If that is the case, it is not clear why Kant needs a notion of purposiveness as design or indeed strong theological commitments that would guarantee the unity of freedom and nature in the first place. Hence, his account would appear to be less prone to a slip into dogmatic metaphysics and more easily reconcilable with the tenets of critical philosophy.

3. The epistemic status of practical ideas

According to Ypi’s interpretative stance, the ‘architectonic unity of the system is thus given by our knowledge of the final end of practical reason that, as Kant argues in the architectonic, guarantees a passage from the realm of nature to that of freedom’ (AR, p. 16). In the preceding section, I take myself to have questioned the claim contained in the latter part of this sentence. Given the weak modal connection between nature and freedom Kant is bound to make good on, it is questionable whether he really needs the notion of purposiveness as design. In other words, the price to pay for architectonic unity may not be quite as high as Ypi has it and not commit Kant to the kind of theology she ascribes to him.

I shall now approach the same question from a slightly different (if closely related) perspective, the epistemic status of practical ideas. Ypi addresses this issue in the former part of the sentence quoted above, where she refers to ‘knowledge’ of the final end of practical reason. If that were in fact what Kant is after, the prospects of delineating his unification project from that of dogmatic metaphysics would be bleak indeed. But is it? It is noticeable that Ypi provides very little explicit discussion of epistemic or propositional attitudes we may adopt in relation to the ideas. This is all the more surprising given that, in the Canon, Kant actually dedicates two subsections to the notions of hope (Second Section, ‘On the ideal of the highest good, as determining ground of the ultimate end of pure reason’) and faith (Third Section, ‘On having an opinion, knowing, and believing’) respectively. Crucially, both epistemic attitudes are justified on distinctly practical grounds (i.e. held in full awareness that we lack epistemic grounds) and thus explicitly delineated from knowledge.

Let me address the notion of hope first. Early on in AR (p. 26), Ypi reminds us that Kant presents his defence of the unity of reason in the Architectonic as a further elaboration ‘of three fundamental questions: “What can I know? What should I do? What may I hope?”’ (A805/B833). While the notion of hope is thus apparently of great systematic significance to Kant’s philosophical thinking in general and the unity of reason in particular, Ypi largely bypasses it. Of course, Kant himself never provides an account of the nature of hope. Hence, interpreters profoundly disagree what it is to hope according to Kant, suggesting for instance that hope is a feeling (Zuckert Reference Zuckert, Sorensen and Williamson2018) or instead something more akin to what most contemporary philosophers take hope to be, a kind of compound state that includes a cognitive element (Chignell Reference Chignell, Barnhill, Budolfson and Doggett2018). For Kant, the question of hope is primarily a normative question – a question concerning what it is permissible or rational to hope for. In the context of the first Critique, he foregrounds the Highest Good as the appropriate object of hope. For reasons of motivation, rational consistency or moral psychology (recall my previous discussion), we must at least be able to hope for a world in which virtue and happiness are in perfect proportion. Indeed, Kant a little later rephrases the question as ‘If I do what I should, what may I then hope?’ (A805/B833). In other words, if I make myself worthy of happiness, I may hope to partake in it.

Two aspects of this argument are worth highlighting. First, hope is justified on distinctly practical grounds: it is our duty to promote the highest good itself, which provides our licence to hope for it. This is confirmed also when Kant highlights that the justification of hope ‘comes down to the inference that something is … because something ought to happen’ (A806/B843). The fact that the notion of hope so obviously concerns the relation between (or unity of) theoretical and practical reason, I take it, would suffice for it to deserve a (more) prominent role in the context of the interpretative line of argument developed in AR. Second, Kant agrees with contemporary philosophers that hope represents its object at least as possible (but not certain). On this basis, Kant distinguishes hoping from mere wishing, which is not connected to a representation of its object as possible, e.g. in the Doctrine of Virtue (MM, 6: 482). That said, the primacy of practical reason does not go so far that we can simply assume the highest good to be possible, simply because it cannot be ruled out as (logically or metaphysically) impossible. Instead, Kant holds that hope requires a ground, i.e. that we must be able to point to some feature of the world indicating that and how its object is possible. This leads us from hope to faith as a second kind of propositional attitude he vindicates in relation to practical ideas.

Faith or practical belief (Vernunftglaube) is at the centre of the Canon’s discussion, mentioned previously, of the difference between opining, knowing and believing (A820–31/B848–59). There, Kant offers a general scheme that allows us to distinguish different positive propositional attitudes – ways of assenting or ‘holding something for true’ (Fürwahrhalten) – and to determine their respective warrant according to whether the ‘subjective’ grounds (i.e. how firmly we hold the assent) and ‘objective’ grounds (how probable the proposition actually is on those grounds) are sufficient. The thought is that, depending on the (subjective and objective) circumstances in which we find ourselves, various kinds of assent of differing confidence and strength are rationally warranted.

For example, an opinion is held weakly, for we recognize that our objective grounds are limited. Knowledge, by contrast, is sufficient from both a subjective and an objective perspective (A822/B850). The most significant punchline of Kant’s discussion, though, is that there is a particularly strong kind of assent – faith or ‘practical belief’ – which we are licensed to hold without sufficient objective grounds but on the basis of practical needs. This is precisely the propositional attitude we are licensed to adopt in relation to the traditional metaphysical objects of God and immortality: given that they are beyond the bounds of sensible experience, they can never be candidates for knowledge, but for the very same reason, we may adopt them on practical grounds. In other words, when we act in pursuit of a practically necessary end, we are licensed to firmly accept a proposition even though we lack conclusive evidence for or against its truth. This, then, is the point where Kant makes good most explicitly on his famous announcement earlier in the Preface that he seeks to ‘deny knowledge in order to make room for faith’ (B xxx).

The relation between hope and faith is a vexed question that Kant never explicitly addresses. That the two attitudes are closely related is beyond doubt. Unfortunately, however, interpreters often simply equate the two (e.g. Flikschuh Reference Katrin, Stolzenberg and Rush2010). Plausibly, we should think of faith as functioning as the ‘cause’ (Ursache) of our hope (A809/B837). That is to say, our faith provides an account of the (explanatory) grounds of possibility of the hoped-for object (cf. Chignell Reference Chignell, Barnhill, Budolfson and Doggett2018; Blöser Reference Blöser, Blöser and Stahl2018). Specifically, our hope for the highest good is sustained by a faith in God and our own immortality (CPrR, 5: 107–48). While we require an immortal soul in order to infinitely approach our own complete conformity with the moral law, the existence of a being with the power to synthesize happiness and virtue is a condition of possibility of a moral world.Footnote ¹ Together, these postulates give us grounds to hope that, by doing ‘our part’ within our lifetime, we actually promote the highest good. While we may not know whether the external circumstances are such that we can contribute to an outcome we ought to bring about, we can accept that this is the case on practical grounds. What warrants our hope is the faith in the conditions on which the realization of our ultimate end depends.

I take it that the emphasis on faith and hope as propositional attitudes that, in being practically justified, are emphatically distinguished from knowledge, throws a different light on the promise of Kant’s endeavour to unify reason in the first Critique. For it leaves space for a reading according to which the Doctrine of Method builds from the materials previously assembled what Kant describes as a stable building of systematic knowledge (A707/B735), without falling into the metaphysical trap of dogmatism. Against this background, the destiny of his unification project in the first Critique (at least from a practical perspective) does not appear quite as bleak as Ypi has it.