This article examines three moments of the post-Kantian philosophical tradition in the nineteenth and early twentieth centuries: Kantianism, Post-Kantian Idealism, and Neo-Kantianism. It elucidates the distinctive methods of a tradition that has never entirely disappeared and is now acknowledged once again as the source of contemporary insights. It outlines two problematics—naturalist scepticism and historicist nihilism—threatening the possibility of metaphysics. The first concerns sceptical worries about reason, emerging from attempts to extend the methods of natural science to the study of human beings. Kant’s project of a critical and transcendental analysis of reason, with its distinctive methods, should be considered a response. The second arises from the development of new methods of historical inquiry, seeming to undermine the very possibility of individual agency. Also considered are Kant’s successors’ revisions of the critical and transcendental analysis of reason, undertaken to overcome challenges confronting the original versions of Kant’s methods.