In what follows I deal with one single thesis that may not be at the centre of Patricia Kitcher's new book (2011) but that is important enough for Kant's philosophy to merit a detailed discussion. Kitcher argues for this thesis concerning the relationship of theoretical and practical reason in Kant in the short but dense chapter 14, ‘Is Kant's Thinker (as such) a Free and Responsible Agent?’ In this chapter, Kitcher on the one hand claims (quite convincingly in my opinion) that the thinking subject is not responsible for its acts of thinking in the moral sense. Those who fail at mental arithmetic miss the truth, but they do not violate an existing obligation. On the other hand, she agrees with Onora O'Neill who in her commentary to Kant's Groundwork (1989) argues that the categorical imperative is the highest principle of both practical and theoretical reason. Kitcher writes:

What does it mean that the categorical imperative is the highest principle of both practical and theoretical reasoning? And what additional support does Kitcher offer for this view?

Let us begin with the first question. Kant takes up the problem of the unity of pure practical and speculative (or theoretical) reason in the preface to the Groundwork of the Metaphysics of Morals by indicating that he is neither willing nor able to solve it in this work. His explanation is:

He had abstained from such ‘completeness’ in the Groundwork, because this would require ‘considerations of a wholly different kind’ that would confuse the reader. These statements seem to make it clear to the reader that in the Groundwork, Kant will not prove that the categorical imperative is the highest principle of reason in general. But this of course does not mean that reason is not the sought uniform principle of pure practical and speculative reason.Footnote ² This just seems to be Onora O'Neill's thesis Patricia Kitcher agrees to.

To understand this thesis better it may be helpful to consider it in some more detail. O'Neill argues as follows. In the Critique of Pure Reason, Kant has established that reason itself must be justified in view of its dialectical entanglements. This can only succeed with a critique by which reason disciplines itself: ‘the authority of reason can only be established by autonomous self-discipline in thought and action’ (O'Neill Reference O'Neill1989: 57). O'Neill thus identifies self-referential critique of reason with the concept of autonomy. Since the categorical imperative is the highest principle of autonomy, as Kant has established in the Groundwork, it is at the same time the highest principle of both pure practical and theoretical reason. It is the highest principle of both acting and thinking. O'Neill writes:

It is easy to see that O'Neill's thesis crucially depends on identifying the concept of critique with that of autonomy. If, first, critique and autonomy are synonyms and, second, the categorical imperative is the highest principle of autonomy, it follows that the categorical imperative also is the highest principle of critique. The fact that the Critique of Pure Reason deals with theoretical or speculative reason as well proves that the categorical imperative is the highest principle of the authority of reason as a whole.

But is it justified to identify critique and autonomy? One way of justifying it could possibly be to refer to those passages in the Critique of Pure Reason (and other writings) where Kant deals with the normativity of theoretical reason. Precisely this is O'Neill's strategy. She points to a passage in the Doctrine of Method in the first Critique where Kant argues:

This passage, according to O'Neill, ‘is the key to Kant's account of the authority of reason. … Reason's discipline cannot be alien; it must be autonomous’ (O'Neill Reference O'Neill1989: 57).

What should we think of this account? O'Neill is certainly right: pure reason is disciplined by pure reason – and not by any other faculty. Because it is a case of self-disciplining one could call it autonomous. Reason disciplines itself by way of critique. But how does this concern moral autonomy in the sense of the self-legislation of our volition? I suppose that O'Neill would point to the word obliegt, that is translated by reference to ‘duty’ in the English edition she quotes. But obliegen does not necessarily indicate anything that involves duty, at least not in the sense of a moral duty. Obliegen can also mean ‘to have as a task’. The task pure reason has to fulfil is to discipline itself. Evidently, this is normative, but it does not seem to force us to believe that this form of normativity is grounded in the categorical imperative and in human volition. Quite the contrary, neither the concept of duty, nor that of the moral law, of autonomy or the categorical imperative are mentioned in the entire section on the Discipline of Pure Reason. For various reasons, this is not a real surprise. Kant does not mention the moral concept of autonomy prior to the Groundwork and he closely connects it to his idea of respect (put in a formula: self-legislation plus respect equals autonomy). In particular, it would be fatal for his entire project of a critique of human reason to link it to the categorical imperative and to autonomy. Why is that? Autonomy is, according to the Groundwork, ‘the will's property of being a law itself’ (4: 447). If autonomy in the sense of moral self-legislation of the will were at the same time an attribute of theoretical reason, this would eliminate any difference between cognition and volition. In the final analysis, all reason would turn out to be pure practical reason. But this is what Kant implicitly rejects when he refers to the difficult problem of the unity of pure practical and theoretical reason.Footnote ⁴ Thus to argue, with O'Neill and Kitcher, that critique and autonomy are fundamentally identical is equivalent to arguing that all reason is pure practical reason. If this were so, however, Kant could have sufficiently informed his readers about this fact with just one sentence in the preface to the Groundwork. But this sentence does not exist, and it does not exist for a good reason, since Kant interprets the problem of how to determine the relationship of theoretical and practical reason as a problem of the system of our higher cognitive faculties. The categorical imperative is not a part of the solution but a part of the problem of a system of our higher cognitive faculties. And the problem increases with Kant's project of a ‘Critique of Taste’ which he pursued since 1786 because he now does not only have to establish the unity of practical and theoretical reasoning but also to integrate reflective judgement and its system of purposiveness into his system of the higher faculties of reason. By his own account, he does not accomplish this task prior to the Critique of the Power of Judgement. Accordingly, Kant argues in the First Introduction to the Critique of Judgement: ‘Philosophy, as a doctrinal system of the cognition of nature as well as freedom, does not … acquire a new part’ (20: 205) by the critical investigation of judgement, but would be incomplete without it as a critical system of philosophy. In other words, if we want to resolve the problem of the unity of pure practical and theoretical reason expounded in the preface to the Groundwork, we have to study the Critique of the Power of Judgement. We have to turn to Kant's conception of reflective judgement and its principle of purposiveness with which Kant wants to accomplish the feat of correlating nature and freedom. But neither O'Neill nor Kitcher mention the Critique of the Power of Judgement at all.

Before completing my first point I would like to add two further observations. First, in the Critical Elucidation of the Analytic of Pure Reason in the Critique of Practical Reason Kant deals with the systematic relationship between the ‘analytic of pure theoretical reason’ and ‘pure practical reason’ (5: 89ff.). However, there is no hint here that the categorical imperative is the highest principle. Second, there is no mention of the categorical imperative as the highest principle in the so-called First Introduction to the Critique of the Power of Judgement, titled On the System of the Higher Cognitive Faculties, which Grounds Philosophy (20: 201–5). To put it another way, if the O'Neill-Kitcher thesis applied, then it would be striking that Kant does not mention it in the two passages discussed.

My second question is, what additional arguments does Patricia Kitcher offer in support of the thesis that the categorical imperative is the highest principle of our entire use of reason? Before I turn to this question I would like to discuss a passage from the third section of the Groundwork by way of introduction that seems to bolster the O'Neill-Kitcher thesis. In this passage, Kant argues that pure practical reason obliges the philosopher, or pure theoretical reason, to free themselves from their apparent dialectical contradictions. Kant writes:

Practical philosophy demands speculative philosophy to resolve the antinomy of pure reason, or else practical philosophy would be vulnerable to sceptical and empiricist attacks. But in my opinion the procedure theoretical philosophy has to choose for this purpose has nothing to do with the categorical imperative and autonomy in the moral sense of the word. How could it? Theoretical philosophy can hardly make use of those weapons it is supposed to sharpen first and foremost. If practical reason achieving ‘tranquillity and security’ on its part requires that pure speculative reason has come to terms with itself, then the criteria for speculative reason accomplishing this task naturally have to be criteria of speculative reason and not of pure practical reason. Thus if Kant speaks of an obligation to self-disciplining in the section on the Discipline of Pure Reason in the first Critique, this would not really support the thesis that the categorical imperative is the highest principle of practical and theoretical reason, even if obligation were understood in the sense of a moral duty. Because self-disciplining and critique of our theoretical use of reason have to follow their own principles rather than those of pure practical reason, I can be morally obliged to deliberate regarding certain things in order to be able to fulfil my duty. Maybe I can also be obligated (as Kant maintains) to inform myself of the limits of my cognitive capacity. But examining my cognitive capacity does not happen in accordance with the criteria provided by the categorical imperative. It concerns cognition and not volition.

I have discussed this passage from the Groundwork because it also seems to be relevant for evaluating Kitcher's ‘additional evidence’. Kitcher characterizes Kant's position as follows: His ‘view is that cognizers must use their cognitive faculties in accord with the moral law’ (Kitcher Reference Kitcher2011: 247), and she refers to passages from the Jäsche logic where Kant elaborates a system of epistemic failures. Reading these passages yields the following, according to Kitcher: ‘From the perspective of his ethical theory, it is clear why the faults of inattention and indulgence are imputable. The individual fails to do his duty, because he is not developing – or even exercising – his talents’ (Kitcher Reference Kitcher2011: 247). I fully agree with Kitcher: of course failures and negligence in thought can be attributed morally if this is the reason why we do not fulfil our moral duties. I could have fulfilled my duties, had I only taken the pains of reflecting on the proper means and ways of fulfilling them. If a physician reaches for a poison instead of a medicine, he is morally accountable for his action. Maybe I am also accountable for those morally required actions I cannot execute just because I have failed to endeavour to practise them. All this holds true, in my opinion. But how is this insight concerning the nexus of thinking and moral duties related to the problem of the unity of pure practical and theoretical reason, as Kant has framed it in the preface to the Groundwork? My answer is that this insight does not concern this problem at all, because the latter is located on a different level. My impression is that my difficulties with Kitcher's chapter 14 rest on the following. On the one hand, Kitcher is right that moral accountability requires a moral perspective that is not given by theoretical reason as such. On the other hand, however, she shares O'Neill's thesis that the categorical imperative is the highest principle of our practical and theoretical use of reason. I just do not see why Kitcher supports O'Neill here. If the ‘additional evidence’ from the Jäsche logic mentioned above would accomplish what it is supposed to accomplish, Kant could have solved the unity problem in an instant, at the latest in the passage of the Groundwork where he emphasizes our duty to cultivate our talents and abilities.

I would like to briefly summarize my remarks as follows:

1. 1. The thesis that the categorical imperative is the highest principle of both pure practical and theoretical reason amounts to claiming that critical self-disciplining has to be considered as an autonomous procedure.

2. 2. This thesis is not persuasive since it hinges on an illegitimate identification of critique and moral autonomy. There is no indication in Kant's writings that he held such a view.

3. 3. Our duty to inform ourselves about the limits of our cognitive capacities, to cultivate ourselves and to seek after truth does not sufficiently ground the thesis that the categorical imperative (in the sense of the unity problem explained in the preface to the Groundwork) is the highest principle of the entire application of our reason. Kant will not address this problem prior to the Critique of the Power of Judgement.Footnote ⁵