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.

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.