 All this leaves the question of the substantive demarcation of duty and supererogation unsettled. Those who deny the existence of supererogation are not bothered by the issue. Those who explain it in terms of exemptions and excuses can appeal to cost-benefit analyses of the enforcement of high standards of behavior on morally weak human beings. But for those who ground supererogation in the intrinsic value of individual autonomy and altruistic intention, personal concern and the expression of virtue, there are no easy criteria for establishing the limits of duty and the space of the supererogatory. One method of drawing this line is phenomenological, that is to say to proceed from “the inside of the agent” and her experience which attests to the difference between the sense of external requirement and the free choice of the individual (Horgan and Timmons 2010). However advocates of this method are fully aware that it can at most serve as an empirical support to the possibility of supererogation, but not as its philosophical justification.