Kant’s Metaphysics of Morals addresses the issue of how the categorical imperative is to be applied by human beings. This work is divided into two parts: the „Doctrine of Right”, in which Kant expressed much of his political philosophy, and the „Doctrine of Virtue”, which is concerned exclusively with other aspects of his moral philosophy. Ostensively, the basis for Kant’s division of work is the difference between duties of justice, juridical (legal) duties, and duties of virtue, ethical duties. Kant says that all legislation (juridical and ethical) consists of two elements: the law, and the incentive to obey the law.
Although two different types of legislation may agree about the law, e.g. both juridical and ethical legislation may prohibit murder, they can at the same time differ with respect to their incentive:
If legislation makes an action a duty and at the same time makes this duty the incentive, it is ethical. If it does not include the latter condition in the law and therefore admits an incentive other than the Idea of duty itself, it is juridica. (MM, p. 19)
Kant is distinguishing ethics, according to which one must always do one’s duty exactly because it is one’s duty, and never for any other reason, from jurisprudence, according to which one can have some other incentive for one’s actions that fulfill one’s duties. In fact, Kant begins the „Doctrine of Right” by defining jurisprudence as „the body of those laws that are susceptible of being made into external laws, that is, externally legislated” (MM, p. 33). Thus, one must determine exactly what laws can be externally legislated.
First we should consider the difference between perfect and imperfect duties, and whether the laws that correspond with each of these can be externally legislated. Perfect duties require the performance or nonperformance of specific actions. It is easy to imagine how laws corresponding to these duties could be externally legislated, as it is easy to imagine how external incentives could be employed to coerce people either to perform or to refrain from performing specific actions.
It seems impossible, however, for external incentives to be employed to coerce people to perform imperfect duties (MM, p.45). After all, imperfect duties require people to adopt certain principles (e.g. beneficience), and to act on those principles in circumstances that they deem appropriate. But people cannot be coerced either to adopt principles or to act on them. Since people’s wills are always free, and adopting a principle is an act of will, people cannot be forced to adopt principles. Furthermore, although people can be coerced to perform acts that follow from specified principles, since the act is merely a response to the coercion, it cannot be construed as an attempt to act in accord with the specified principle. Thus, people cannot be forced to act on a principle. Therefore, only imperfect duties can be subject matter of jurisprudence, imperfect duties cannot.
It might appear, then, that all perfect duties can be addressed by jurisprudence. Actually, however, there is good reason to believe that Kant means to exclude perfect duties to oneself from the domain of jurisprudence. One clear statement to this effect follows: „The concept of justice … applies only to the external and – what is more – practical relationship of one person to another„(MM, p. 34).
(Kant’s moral and political philosophy in The Age of German Idealism (coord. R. C. Solomon, K. M. Higgins), Routledge History of Philosophy, vol. 6, 2003, la pp. 83-84)