 Although earlier philosophers, including Socrates, appealed to something resembling the principle of fairness (or fair play), the classic formulation of the principle is set out in H. L. A. Hart’s “Are There Any Natural Rights?” As Hart there says, “when a number of persons conduct any joint enterprise according to rules and thus restrict their liberty, those who have submitted to these restrictions when required have a right to a similar submission from those who have benefited by their submission” (1955, p. 185). John Rawls subsequently adopted this principle in an influential essay of his own, referring to the duty derived from the principle as the “duty of fair play” (1964). What the principle of fair play holds, then, is that everyone who participates in a reasonably just, mutually beneficial cooperative practice – Hart’s “joint enterprise according to rules” – has an obligation to bear a fair share of the burdens of the practice. This obligation is owed to the others who cooperate in the enterprise, for cooperation is what makes it possible for any individual to enjoy the benefits of the practice. Anyone who acts as a free rider is acting wrongly, then, even if his or her shirking does not directly threaten the existence or success of the endeavor. Those who participate in the practice thus have rights against as well as obligations to one another: a right to require others to bear their share of the burdens and an obligation to bear one’s share in turn.