In Chapter 14 we established the foundation for the human being's duties, rights, and moral faculties. Having a duty means that one is required to commit or omit a specified action. “Duty is that action to which someone is bound.” Duty is predicated on the presumption that the individual who has the duty also has the ability to fulfill its requirements. In this Appendix we briefly discuss the well-known “‘ought’ implies ‘can,’” which, although often attributed to Kant, is indeed older.

The implication contained in “‘ought’ implies ‘can’” can be seen from both a prospective and a retrospective view of the actions we commit or omit. We begin with the prospective and consider rules of conduct functioning to model behavior. From this point of view, a rule tells us what we ought to do in the future (section 1). Inextricably connected to this function is the rule's function to evaluate behavior. In the retrospective, the rule sets the standard for evaluating our past conduct (section 2). From either point of view, when we apply a rule to an act we assume that the actor can act, or could have acted, according to the rule's requirements.