In a theory of justice Rawls distinguishes “natural duties” from obligations: natural duties are incumbent upon each of us unconditionally, whereas obligations are voluntarily incurred. He also distinguishes natural duties from duties that are attached to institutional offices or other social positions. Natural duties are moral requirements. As the institutions and social positions to which institutional and social duties attach may be morally defensible or indefensible, they do not necessarily possess any moral force (TJ 98f.).

Rawls acknowledges a diverse set of natural duties, including “the duty of helping another when he is in need or jeopardy . . . ; the duty not to harm or injure another; . . . the duty not to cause unnecessary suffering” and the duty of mutual respect (TJ 98, 297).Within the context of a theory of justice, however, the most important natural duty requires us “to comply with and do our share in just institutions when they exist and apply to us” and “to assist in the establishment of just arrangements when they do not exist” (TJ 293f.).

Rawls holds that the parties in the “original position” would endorse this natural duty of justice as “the easiest and most direct way” “to secure the stability of just institutions” (TJ 295). By contrast, the principle of fairness, which in effect supplements the duty of justice for those who “gain political office” and “take advantage of the opportunities offered by the constitutional system,” provides less support for just institutions (TJ 302f.).This represents a departure from his earlier position, which was that mere receipt of benefits from mutually beneficial and just social arrangements grounds amoral obligation of obedience to law (CP 117–128).