To understand why rational thinkers, according to Kant, are naturally led to accept the Supreme Principle as true, we must answer two questions: why does the Logical Maxim have to become a principle of pure reason – that is, why is it rationally necessary to make regulative use of the Supreme Principle? And why does this lead to the illusion that the Supreme Principle is an objectively valid constitutive principle (i.e., a true descriptive statement about everything there is)? Chapter 5 offers answers to these questions. The latter question in particular requires a discussion of Kant’s account of transcendental illusion and the role of transcendental realism in bringing about this kind of illusion. The central idea is that transcendental realism implies that there is a correspondence between reason and reality; therefore, a tacit commitment to transcendental realism can explain why regulative principles of reason will naturally be taken to be constitutive principles that are true descriptions of reality itself. Even though transcendental realism is a weighty metaphysical claim, it can plausibly be attributed to common sense or ‘universal human reason.’ The chapter closes with a discussion of why, according to Kant, the Supreme Principle is valid for things in themselves.

Chapter 1 offers a first and preliminary outline of Kant’s accounts of reason and metaphysics and introduces various themes that will be developed further in later chapters of the book. In particular, Kant’s conception of reason as a cognitive faculty is distinguished from reason as a system of principles and Kant’s distinctions between reason and pure reason, between theoretical and practical reason, and between the logical and the real use of reason is introduced and related to other conceptions of reason from the history of philosophy. Moreover, Kant’s conception of metaphysics as purely rational cognition as well as its major divisions (such as that between metaphysics of nature and metaphysics of morals and between metaphysics as a science and as a natural predisposition) are discussed.

The book closes with a brief Postscript on Kant’s ‘practical metaphysics.’ In the Critique of Practical Reason and elsewhere, Kant introduces three ‘postulates of pure practical reason’, which concern God’s existence, freedom of the will, and the immortality of the soul. We briefly discuss whether reason eventually finds in a practical mode what it had been looking for in a speculative mode, namely, the unconditioned. (The answer is: almost, but not quite.)

Chapter 7 turns to the first two of the three metaphysical disciplines whose inferences and conclusions Kant discusses in the Transcendental Dialectic, namely, rational psychology and cosmology. We show how the four transcendental paralogisms and the arguments underlying the four antinomies of pure reason can be read as ‘necessary inferences of reason’ and how they result in the corresponding transcendental ideas. The chapter considers the paralogisms and the psychological ideas, discusses the role of transcendental realism in the paralogisms, looks at the derivation of the cosmological ideas and the four antinomies, and reconstructs the role of transcendental realism in the antinomies. In doing so, we concentrate on those aspects that are relevant to understanding how Kant develops his Rational Sources Account in the Transcendental Dialectic.

Chapter 3 turns to the fundamental metaphysical concept in Kant, the concept of the ‘really’ (not just logically) unconditioned, and to the conditioning relations between objects that speculative metaphysics tracks. Kant’s Rational Sources Account (his argument that the sources of speculative metaphysics lie in reason itself) rests on the claim that the logical use of reason naturally leads us to accept a principle he calls the “supreme principle of pure reason.” In this chapter, we will try to clarify this principle by asking what Kant, in the context of the Supreme Principle, means by ‘given,’ by ‘the conditioned’ and its ‘conditions,’ what it means for something to be ‘unconditioned,’ and why Kant thinks the series of subordinated conditions is supposed to be unconditioned in this sense. Finally, we will ask how that principle relates to the Principle of Sufficient Reason.

Chapter 2 discusses Kant’s account of the ‘logical use of reason,’ which consists in drawing ‘mediate inferences’ (e.g., syllogisms). It is guided by the ‘Logical Maxim,’ which requires us to search for the conditions of or conditioned cognitions until we arrive at unconditioned cognitions. Its aim is to transform our manifold cognitions about nature (both empirical and a priori) into a complete system of scientific knowledge and thus to achieve the ‘unity of reason.’ The chapter also places Kant’s conception of systematic cognition in the context of the Wolffian school and explains what it means for cognitions to be conditioned or unconditioned. Finally, the Logical Maxim is defended as a plausible expression of our rational striving for systematicity and completeness of knowledge.

A central aspect of Kant’s Transcendental Dialectic is his claim that there are concepts – the transcendental ideas – that necessarily arise from rational reflection. According to Kant, these ideas of reason, like the categories of the understanding, form an a priori system. In Chapter 6, we first look at Kant’s conception of transcendental ideas and survey the system they form. Next, it is argued that Kant does not attempt to derive the transcendental ideas in questionable ways from the forms of rational inferences or the possible relations between subject, object, and representation (even though the text suggests this), but rather considers them, much more plausibly, as concepts we arrive at through rational inferences about specific (psychological, cosmological, and theological) subject matters. The central philosophical point here is that concepts can be the result of (what Kant calls) ‘necessary inferences of reason.’ A first instance of this is Kant’s derivation of the concept of the unconditioned; the chapter then turns to the three classes of transcendental ideas (psychological, cosmological, theological).

Having investigated the Logical Maxim and the Supreme Principle, Chapter 4 turns to the transition from the former to latter. In a suggestive passage from the Introduction to the Transcendental Dialectic, Kant claims that the Logical Maxim can become a principle of pure reason only by our assuming the Supreme Principle. In order to understand this, we look, first, at the final paragraph of the Introduction, where Kant raises the question of whether the Supreme Principle is ‘objectively valid,’ and then at the Appendix to the Transcendental Dialectic, where he also claims of various ‘logical principles’ that they presuppose ‘transcendental principles,’ which we must therefore ‘assume.’ In light of the distinction between the regulative and the constitutive use of principles, there are two very different ways of understanding what it means to ‘assume’ a principle. Our result will be that the transition to the metaphysical Supreme Principle involves two steps not made explicit by Kant: one from the Logical Maxim to the regulative use of the Supreme Principle, and another from the regulative use of the Supreme Principle to its constitutive use.