Kant famously claims that the Categorical Imperative is a single principle which can be expressed in a number of formulations - a claim that has puzzled interpreters ever since. This thesis reconstructs Kant’s argument in light of its historical background. It is argued that Kant views the formulations of the Categorical Imperative as expressions of a single principle grounded in reason. He takes these formulations to be conceptually distinct yet co-extensive in practice. This leads Kant to conclude that the formulations are equivalent. However, it is unclear how this conclusion is supposed to follow. If the formulations are construed as independent moral criteria, it becomes difficult to account for their equivalence. The thesis instead suggests that Kant’s conclusion only seems to follow if we take him to be assuming a logical-metaphysical framework characteristic of his pre-modern predecessors. By recognising how Kant’s argument relies on a principle borrowed from ancient and medieval logic and metaphysics, we get a better understanding of the internal structure of Kantian moral reasoning.