The moral obligation to do or to omit something. This is duty taken subjectively. Understood objectively, duty is the thing that must be done or omitted. The difference is expressed in the two statements "He has a duty," meaning that a person is morally obliged, and "He does his duty," meaning that a person does the thing he is obliged to do.
Rights and duties are correlative and complementary. thus if one person has a right, everyone else has a duty to respect that right. And if one has a duty, one also has the right to fulfill that duty and do all the things that are necessary for its fulfillment; otherwise it would not b e a genuine duty. Nevertheless, while having a right, a person does not necessarily have a duty to exercise that right. In fact, no one can exercise all one's rights but must choose among them, for some of them are simultaneously incompatible, as the right to stand and the right to sit. God is the only exception to the mutual dependence of rights and duties. He has all rights and no strict duties; creatures have duties to him but no rights against him; human beings have both rights and duties toward one another.