The natural duties are those that are binding on individuals without the performance of previous voluntary acts. Among them, Rawls mentions the duties of mutual aid, mutual respect, not to harm or injure another, and not to cause unnecessary suffering. From the standpoint of a theory of justice, the duty of justice is the fundamental requirement for individuals (TJ 296). These duties are “natural” because they are owed to persons generally regardless of any institutional relationship, and their content is not defined by the rules of institutions or of social practices. Rawls contrasts them with “obligations,” which are grounded on the principles of fairness. Obligations, in this sense, are defined by the rules of just (or fair) institutions or practices, they arise as a result of voluntary acts, and they are owed to definite individuals. The natural duty of justice has two parts: “it requires us to support and to comply with just institutions that exist and apply to us”; it “also constrains us to further just arrangements not yet established” (TJ 99). According to this, everyone has a natural duty to do his or her part in a social scheme in which the basic structure is just or “as just as it is reasonable to expect in the circumstances” (TJ 99). Thus, the duty of justice has an important role in making social cooperation stable (TJ 293).