We are far from done, however, with the idea that the solution to the issues discussed in this book is to be found in a direct appeal to cosmopolitan “natural duty,” an appeal that would make any contractualist apparatus (however understood) redundant. That idea finds very strong support. Martha Nussbaum's view – that we must give up on the very idea of a social contract if we are to make sense of our obligations other than to citizens – has already been mentioned above. Jeremy Waldron has proposed that the “special duties” of citizenship are best explained as derivatives of natural duty, as necessary (not contractual) implications of justice–promoting arrangements. Allen Buchanan – one of whose proposals will be examined at length below – argues that only an appeal to a strong “natural duty of justice” can justify one society's coming to the aid of another, and that contractualism implies moral isolationism. But perhaps the most basic case for a direct appeal to natural duty is offered by Liam Murphy in his critique of Rawls. Rawls maintains that we have a natural duty of justice only in relation to associative requirements that “apply to” us. Given that we are part of an institution (a state) that issues binding requirements upon us, we have a duty to assess the justice of its requirements.