Kant does not completely work out his philosophical conception of enlightenment. The definition of enlightenment that he offers in his well-known essay on the topic does not seem to completely match the definition that he puts forward later in his essay on the pantheism controversy and in the third Critique. It remains unclear how the two definitions relate to each other and whether and how they rest on the same principle. The lack of clarity in Kant’s conception of enlightenment is left mostly unaddressed in the secondary literature on the topic. This paper suggests that Kant’s philosophical conception of enlightenment rests on the discipline of pure reason. The self-disciplinary principle of reason, which is laid out in the Doctrine of Method of the first Critique, underlies both of Kant’s definitions of enlightenment, although he himself does not clearly demonstrate how it does so.