The article examines the influence of German philosopher Gottfried Leibniz on pragmatist Charles Peirce's understanding of Enlightenment philosopher Immanuel Kant. More specifically, the author explains Peirce's combination of rationalist dogmatism with Kant's philosophy, as well as Peirce's understanding of Kant's transcendental thought and concept of the noumenon. It is argued that while Peirce supported transcendental philosophy in general, he rejected the noumenon and obscured the boundary between a priori and a posteriori knowledge because Kantian transcendentalism did not fit in with his own metaphysical philosophy.