Professor James Urmson of Stanford University, Philosophy Dept., writes
If we are to exact basic duties like debts, and censure failures, such duties must be, in ordinary circumstances, within the capacity of the ordinary man. It would be silly for us to say to ourselves, our children and our fellow men, 'this and that you and everyone else must do,' if the acts in question are such that manifestly few could bring themselves to do them, though we may ourselves resolve to try to be of that few. To take a parallel from positive law, the prohibition laws asked too much of the American people, and were consequently broken systematically; and as people got used to breaking the law a general lowering of respect for the law naturally followed; it no longer seemed that a law was something that everybody could be expected to obey. Similarly in Britain the gambling laws, some of which are utterly unpractical, have fallen into contempt as a body. So, if we were to represent the heroic act of sacrificing one's life for one's comrades as a basic duty, the effect would be to lower the degree of urgency and stringency that the notion of duty does in fact posses. The basic moral code must not be in part too far beyond the capacity of the ordinary men or ordinary occasions, or a general breakdown of compliance with the moral code would be an inevitable consequence; duty would seem to be something high and unattainable, and not for 'the likes of us.'