Can Kant's ethical scheme accommodate the supererogatory?
If obligatory actions are those that one is duty-bound to perform, a supererogatory action is one that is above and beyond the call of duty. Michael A. Monsoor's throwing himself on a live grenade to save his Navy SEAL buddies is a paradigmatic example. But in a wide sense, a supererogatory act is any act, however trifling, that is in excess of what is morally required, any act that is morally good but the nonperformance of which is not morally bad.
One idea worth exploring is that there is room for supererogatory acts in Kant's scheme under the rubric of imperfect duties. Kant tells us that ". . . by a perfect duty I here understand a duty which permits no exception in the interest of inclination . . . ." (Grundlegung zur Metaphysik der Sitten, AA 421, fn) A perfect duty can be either towards oneself or towards another. Kant gives the prohibition of suicide as a perfect duty to oneself and the prohibition of deceitful promises as a perfect duty towards others. These are proscriptions that admit of no exception, and I take it that perfect duties are perfect in that they are exceptionless: in no circumstance may one take one's own life, and in no circumstance may one make a deceitful promise. If so, then imperfect duties are prescriptions that admit of exceptions. This interpretation fits the examples Kant gives (423-424). There is the duty to oneself of cultivating one's talents, and the duty to to others of benevolent assistance. Both of these duties are prescriptions and both admit of exceptions. My general duty to cultivate my talents is not a duty to cultivate all my talents, or even any particular talent. And the duty of benevolent assistance cannot be a duty to assist everyone. Paul Guyer's reading is similar:
In the Groundwork, Kant's principle of morality gives rise to a fourfold classification of duties, resulting from the intersection of two divisions: between duties to oneself and to others, and between perfect and imperfect duties. Perfect duties are proscriptions of specific kinds of actions, and violating them is morally blameworthy; imperfect duties are prescriptions of general ends, and fulfilling them by means of performing appropriate particular actions is praiseworthy. The four classes of duty are thus: perfect duties to oneself, such as the prohibition of suicide; perfect duties to others, such as the prohibition of deceitful promises; imperfect duties to oneself, such as the prescription to cultivate one's talents; and imperfect duties to others, such as the prescription of benevolence (4: 422-3, 429-30 ). It is straightforward what a perfect duty prohibits one from doing; it requires judgment to determine when and how the general ends prescribed by imperfect duties should be realized through particular actions.
Note that Guyer states that the fulfilling of imperfect duties is praiseworthy. Now it is the supererogatory that we praise: it is inappropriate to praise people for doing what they are obligated to do. How morally absurd to praise a pater familias for paying the rent and putting food on the table! That is what he is supposed to do, what he morally must do. The failure to do such is blameworthy, but the performance is merely required, hence not praiseworthy. Since the fulfilling of imperfect duties is praiseworthy, it seems we can conclude that in Kant the class of supererogatory acts either is or is a proper subclass of the class of imperfect duties.
Further support for this interpretation comes at Grundlegung 429-430 where Kant speaks of "necessary or obligatory duties to others" and a "contingent (meritorious) duty to oneself." In English, 'obligatory duty' smacks of pleonasm, but that might not be the case for Kant's schuldige Pflicht. If duties divide into the obligatory (schuldigen) and the meritorious (verdienstlichen), then we can say that Kant accommodates supererogatory acts under the rubric of imperfect or meritorious duties.
There is more to it than this, of course, and there is a technical literature on this topic only a small amount of which I have read; but I think I can safely say two things: (i) a case can be made for Kant's being able to accommodate the supererogatory, and (ii) Kantians are more hospitable to the supererogatory than are utilitarians.