My thesis offers a new perspective on Kant’s theory of reason. I start from the observation that Kant characterises reason as an infinitely demanding faculty. For Kant, reason not only demands that we draw valid inferences and obey the laws of logic, but being governed by a version of the PSR, reason, also demands that we find complete explanations for all observable phenomena, thereby realising the complete system of all cognition in science. This account of reason is no philosophical faux pas, but underpins Kant’s practical metaphysics. For example, Kant thinks that we must rationally believe in our own immortality, or else we couldn’t realise reason’s infinite demands.

This observation prompts the main question of my thesis: How does Kant justify the existence of such a faculty in the first place? How does he think we, as finite beings, can come to know that we have an infinitely demanding faculty of reason? Traditionally, Kant scholars have held that we can know our mental faculties, in general, either by being aware of their unique acts or by transcendental arguments. Both strategies, however, fail in the case of reason. We cannot be aware of reason’s unique acts, which are infinite and thus never complete; and reason cannot be established via transcendental argument because it is not a necessary ingredient for the possibility of experience.

But how then can we know that we have reason? My thesis breaks with a long-held orthodoxy and argues that reason gains self-knowledge via empirical psychology (i.e., the systematic study of phenomena in inner sense). The overarching argument goes like this: Reason, for Kant, seeks to explain all phenomena, including those of inner sense. To explain inner phenomena, reason hypothesises mental faculties and their laws. Our inner urge to ask why-questions, Kant thinks, is best explained by hypothesising a faculty that demands complete explanations, i.e., reason. In empirical psychology, reason therefore comes to know itself as faculty that seeks complete explanation.

My thesis is divided into five chapters. I first show that, for Kant, mental faculties are (also) powers of inner sense (chapter 1), and that the normative demands of these faculties are grounded in constitutive principles or laws (chapter 2). I then argue that the constitutive principle of reason, to find complete explanations, demands that we systematise powers of nature (chapter 3), which we do by hypothesising their respective laws (chapter 4). Bringing it all together, I then argue that reason hypothesises its own explanation-seeking law as the best explanation of phenomena in inner sense, thereby discovering its own infinitely demanding nature (chapter 5).