Kenneth Einar Himma (University of Washington - School of Law) has posted The Logic of Showing Possibility Claims: A Positive Argument for Inclusive Legal Positivism and Moral Grounds of Law (REVUS: Journal of Constitutional Theory and Philosophy of Law, Forthcoming) on SSRN. Here is the abstract:
In this essay, I argue for a view that inclusive positivists share with Ronald Dworkin. According to the Moral Incorporation Thesis (MIT), it is logically possible for a legal system to incorporate moral criteria of legality (or “grounds of law,” as Dworkin puts it). Up to this point, the debate has taken the shape of attacks on the coherence of MIT with the defender of MIT merely attempting to refute the attacking argument.
I give a positive argument for MIT. I begin with an explanation of the logic of establishing possibility claims, such as MIT. At the outset, it is worth noting that the logic of establishing possibility claims is very different from the logic of establishing contingent descriptive claims or necessary claims. For this reason, some explication of the relevant features of the semantics of modal logic will be necessary here. Once the structural framework is adequately developed, the argument for MIT will be grounded on the strength of a thought experiment of a surprisingly simple kind. Indeed, the argument is inspired by a Razian argument for the possibility of a legal system without coercive enforcement machinery; on his view, a society of angels could still have a system of law without any coercive machinery. My argument will possess two theoretically important qualities that are also possessed by Raz’s powerfully simple, but ultimately unsuccessful, argument.
And from the paper, the passage in which the thought experiment is laid out:
[T]here is a conceptually possible world in which everyone is infallible in discerning the requirements of morality but frequently acts in ways contravening those requirements in order to further his or her own selfish interests. Further, in one such world, an institutional system of rules is set up to regulate the behavior of citizens who act out of selfish interest when it conflicts with morality. Moreover, the grounds for membership in this institutional system of rules are exhausted by the norm all and only moral norms constitute a rule of the system. The fact that everyone is morally infallible means that everyone always knows what the law requires with respect to any particular act.