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.