A duty is the obligating or binding force of law upon a person — what one is required by law to do or forbear. Salmond: a duty is an act which is in accordance with law and is enforced by law... more precisely, a duty is the correlative of a right: the burden of the right.
Classifications:
1. Legal and moral duties — Legal duties are enforceable by courts; moral duties (gratitude, charity, filial respect) are enforced only by conscience and society. A legal duty may have no moral force (a harsh contract term) and a moral duty may have no legal force (rescuing a drowning stranger). Some duties are both — the duty not to murder.
2. Positive and negative duties — to act (pay tax, maintain a parent under the Criminal Code's maintenance provisions) versus to forbear (not to defame, not to trespass).
3. Absolute and relative duties — According to Austin, duties owed to persons determinate (relative — pay X money) or indeterminate/owed to society at large (absolute — do not murder, do not pollute). Salmond, however, denied that absolute duties exist — every duty, he argued, is owed to some right-holder, even if that holder is the sovereign or the community in its corporate capacity.
4. Primary and secondary duties — a primary duty exists independently (do not assault); a secondary duty is consequential and remedial (the duty to compensate for the assault). The whole law of remedies is the law of secondary duties.
Constitutional dimension — Nepal: Alongside enforceable fundamental rights, the Constitution of Nepal (2015) contains a chapter on the duties of citizens — to safeguard the nationality, sovereignty and integrity of Nepal, to comply with the Constitution and law, to protect public property, and the like. These are in the nature of constitutional-moral duties: they bind as constitutional norms of statecraft, though their direct court enforcement against an individual is rare. Directive principles similarly cast duties on the State rather than rights in citizens (a matter explored in Constitutional Law-I).