The concept of supererogation is an act that it is right to do but not wrong not to do. The moral trinity of the deontic logic excludes such acts from moral theory. A moral theory that is based on duty or obligation unqualified seems inevitably to make all good acts obligations, whether construed from a teleological or deontological point of view. If supererogation is a moral fact, no moral theory can survive without acknowledging it. One way to distinguish supererogation from obligation that is not arbitrary is to draw the line of obligation at death and dismemberment. Such a limit to obligation is often implicit in moral theory. Inclusive obligation requires us all to be heroes all of the time. The moral limit to obligation is one of Hobbes's teachings. Though it is seldom noted in contemporary political and moral theory, it is, for example, implied in Rawls's definition of ‘supererogation.’ In this definition it is said that heroic supererogation would be a duty but for the high cost associated with it. This cost is the risk of life and limb;.it distinguishes supererogation from both benevolence and obligation.