This essay examines one of the most obscure and controversial tenets of Kant’s critical philosophy, his claim in the Critique of Practical Reason that the moral law is immediately and unquestionably valid as an a priori fact of reason (Factum der Vernunft). This argument curiously inverts Kant’s earlier stance in the Groundwork of the Metaphysics of Morals, in which he justifies the reality of the categorical imperative through a much more cautious and qualified authentication of transcendental freedom. Against constructivist readings which tend to downplay the justificatory burden of the Factum, I claim that Kant’s position is best understood as a transcendental argument. In other words, he argues from a given or assured conditioned, consciousness of binding moral obligation, to the sole condition of its possibility in transcendental freedom. In order to rebut the standard objections prompted by this line of interpretation, I emphasize the technical function of Kant’s Factum as both a deed (Tat) and product (Tatsache) of practical reason.