Kant illustrated how to apply the categorical imperative by selecting an example from each of four classes of duties. The four classes are differentiated partly by the traditional distinction between perfect and imperfect duties. But Kant offered no clarification of this distinction in the Groundwork, except to say in a footnote that, as indicated in the quotation above (Kant 1997, 31n/4:421n), perfect duties admit no exception regarding inclination.

Duty and inclination. Kant typically used the word “inclination” when referring to natural or experience-based desires. In an earlier explanation of duty and moral worth he made it clear that acting from inclination is incompatible with acting from duty. But here, when he defines “perfect duties” as duties admitting no inclination-based exceptions, he implies that some duties, namely “imperfect duties,” would in some way yield to inclination. For many readers this raises a puzzle, as expressed in the question once asked by Ross: “what sort of duty would that be, which we are free to do or not to do as we feel inclined?” (Ross 1954, 45). The answer, which Kant did not get around to explaining until writing The Metaphysics of Morals (1797), is that these are “duties of virtue.” They are, technically, duties to have or work toward certain ends: others’ happiness, and self-improvement (Kant 1991, 187-98/6:382-95). Actions directed toward these ends are duties that admit of exceptions in the interest of inclination because we not morally required to do everything we can to achieve them. With respect to others’ happiness, for instance, morality does not require us to sacrifice our own happiness in order to make others happy. This means, therefore, that our own happiness (inclinations) properly limits what we must do for the sake of others’ happiness. We ought, morally, to act in ways that benefit others; but we are free to choose how much and how often, as befits our inclinations.

Perfect and imperfect rights. Beginning over a century prior to Kant’s lifetime, political philosophers like Hugo Grotius (1583-1645) and Samuel Pufendorf (1632-1694) were writing about the distinction between perfect and imperfect rights. The former are rights that should be enforced by civil legislation, they thought; the latter would not be so enforceable, though everyone ought nevertheless to recognize these rights as belonging to their fellow citizens equally, and to do what they can to respect them. Citizens’ perfect rights typically required others, including the government, to respect their personal freedoms and not interfere with their lives, property or activities. Their imperfect rights would require others actively to help them if they may be in need—the poor, particularly, were thought to have imperfect rights to occasional assistance of the more well-off. Grotius thought, for example, that human beings would have chosen to move from the original social arrangement of communal property to one respecting private property only if they could be assured of others’ assistance should they end up without means (Grotius 2005, 433-5). Applying the idea that rights always imply corresponding duties for others, it is easy to see how the distinction between perfect and imperfect duties arises from that between perfect and imperfect rights. Yet Kant offers a somewhat different conception of the duties in question. One difference is that he does not see perfect duties stemming always from others’ rights. By defining them as duties not admitting exceptions in favor of inclination, he is able to acknowledge that we can have perfect duties even to ourselves, as illustrated in the suicide example.

Developing natural talents. Kant’s third example of duties derived from the categorical imperative is the imperfect duty to oneself to develop one’s natural talents (Kant 1997, 32-3/4:422-3). The maxim considered is something like: “I’ll pursue idle pleasures rather than work to improve myself.” Kant claims that a rational being cannot will this maxim as a universal law since: “as a rational being he necessarily wills that all the capacities in him be developed, since they serve him and are given to him for all sorts of possible purposes.” Here again, as in the suicide example, it may first appear that Kant’s rejection of the maxim has nothing to do with the prospect of its universality. Under this illusion, some have criticized Kant for deriving the duty of self-improvement from enlightened self-interest, or an appeal to consequences, which Kant himself claims must be ignored (see Ross, 46-7). But in this case, as with the former, Kant’s focus is instead on the prospect of the maxim’s holding as universal law of nature. He is impressed by the idea that our talents come from nature, purposive nature, and so are by nature meant to be used for the benefit of ourselves and the species. This is an assumption of natural teleology similar to assumptions considered in Kant’s argument about the end of practical reason. Nature has also given us the capacity to feel pleasure and amusement, of course. So the question comes down to this: could a rationally ordered nature give lawful priority to pleasure and amusement over the development of talents? The answer seems pretty clearly to be negative.

Helping others in need. To illustrate an imperfect duty to others Kant considers the maxim of a selfish person who decides never to expend any effort or wealth for the benefit of others in need of assistance. The argument offered here is that his maxim cannot be rationally willed to become a universal law because of the following. The selfish person, pursuing nothing but his own happiness, must realize that sometime he will need the assistance of others in order to attain his goal. If his maxim were a universal law, however, no one would be there to offer that assistance, and he could not achieve his selfish end. In willing his maxim to be universal, “he would rob himself of all hope of the assistance he wishes for himself” (Kant 1997, 33/4:423). This example is related to a way the basic idea of the categorical imperative is expressed a number of years later in the development of Kant’s philosophy.

Assume a human being . . . who allows himself to think (as he can hardly avoid doing) what sort of world he would create, were this in his power, under the guidance of practical reason – a world within which, moreover, he would place himself as a member. (Kant 1998, 35/6:5)

It seems unreasonable to place oneself willingly in a world, of one’s own design, where one’s primary end in that world cannot be achieved—that is what is known as setting oneself up for failure. But there will be those impressed with their own self-reliance who find it perfectly reasonable never to accept anyone’s offer of help, for they can always make do on their own. They should understand, however, that they have not gotten to that position of smug independence without prior help. Their selfish maxim’s holding as a universal law would effectively create a world in which they could have no hope of achieving their current level of competence in society or in nature. If the maxim of never giving aid in general fails the universalization test, could maxims of not giving particular kinds of aid pass the test? (Wood 1999, 96). It seems so. For example, people suffering from end-stage renal failure desperately need others to donate one of their kidneys. Since I have never needed this type aid, and since the odds are overwhelming that I never shall, it seems my maxim never to donate a kidney could hold as a universal law. But it is hard to see how this poses a problem for the formula of universal law. It is hard to see that the formula has gone wrong for allowing such a maxim to pass. It should be noted too that if that maxim would fail, then everyone would have a duty to donate a kidney, which does not seem reasonable. Perhaps there is confusion here over the kind of maxim involved. If a maxim not to do a certain type of action, like donating a kidney, would fail the test, then that would make it a duty to do that kind of action. But when a maxim not to contribute to a certain end fails, the result is an imperfect duty to do something, at one’s own discretion, to advance that end. The challenge posed by the maxim not to offer a certain kind of aid misses the mark, since that maxim focuses on a particular action-type, rather than on contribution to advancing an end.

Duties of virtue. The preceding examples of imperfect duties to oneself and to others are instances of what Kant will later call “duties of virtue.” They are also classifiable as duties of “wide obligation” (Kant 1991, 194-5/6:390-1), which means that, although they are duties to act in order to bring about ends, we need not concentrate all our efforts on fulfilling such duties. We have wide discretion in how to go about fulfilling these duties, and in how hard to try. Virtue comes in degrees, and some will always be more virtuous than others. Those who try harder to improve themselves, or who sacrifice more for the sake of others, are the more virtuous. It is difficult to say at what point one can have “done enough” in the cause of virtue, if one ever can. Kant offers no formula for determining this, except to say that “Virtue is always in progress, and yet always starts from the beginning” (Kant 1991, 209/6:409). Another point worth making about the wide duties of virtue is that they take lower priority, as duties, than “narrow,” or perfect duties. Kant did not make this clear in the Groundwork examples. We have a duty to help others in need, for example, but it would be contrary to duty to make a lying promise in order to do so. So the virtuous end does not, by itself, justify the means taken in order to achieve it. Related to this point, Kant will show later, in The Metaphysics of Morals, how the perfect duties to others correspond to rights. Rights, in the view of deontological thinkers like Kant, take higher moral priority than virtues. Duties corresponding to rights are more stringent than duties of virtue.