Kant’s theory of punishment has been studied without any further consideration of the wider philosophy that grounds it. In this article, I will seek to make explicit Kant’s theoretical/philosophical grounding of the theory. I will intend to exhibit how Kant deduces the categorical character of criminal law from the Principle of Right and the categorical imperative. This theoretical grounding of the categorical character of punishment will hold only insofar as it is possible to understand why Reason is capable of imposing categorical principles of action—i.e. duties—to itself.