In this doctoral thesis, Kant's distinction between perfect and imperfect duties is examined. The thesis begins with an exploration of how the distinction originates and evolves in the writings of three of Kant's most prominent natural law predecessors: Hugo Grotius, Samuel von Pufendorf, and Christian Wolff. The thesis then moves on to Kant's own writings. It is argued that Kant draws the perfect-imperfect distinction in as many as twelve different ways, that these ways are not entirely consistent with one another, and that many of them, even taken by themselves, do not hold up to scrutiny. Furthermore, it is argued that Kant's claim that perfect duties always trump imperfect duties - which can be referred to as "the priority claim" - is n...