The Nash-Harsanyi theory of bargaining is usually taken as the correct theory of rational bargaining, and, as such, as the correct theory for the basic political contract for a society. It grafts a theory of cooperation to a base that essentially articulates the perspective of non-cooperative interaction. The resultant theory is supposed make clear how rational bargaining can fully realize the mutual gains that cooperation can make possible. However, its underlying commitment to the concepts of non-cooperative interaction renders this doubtful. I argue for an alternative theory—the theory of Full Cooperation—that avoids this difficulty. As applied to bargaining over the basic political contract, it calls for the selection of the most egalitarian of the Strictly Pareto-Optimal outcomes. This is essentially a version of Rawls’ principle of Justice as Fairness, but it is derived, unlike Rawls’ theory, without appeal to a problematic theory of individual decision-making behind the veil of ignorance. It also provides an alternative basis for what Hart calls the Principle of Mutual Restrictions, and what Rawls’ calls the Duty of Fair Play.