§ 71 Duties
A duty is an obligatory act, that is to say, it is an act the opposite of which would be a wrong. Duties and wrongs are correlatives. The commission of a wrong is the breach of a duty, and the performance of a duty is the avoidance of a wrong. A synonym of duty is obligation, in its widest sense, although in a special and technical application the latter term denotes one particular kind of duty only, as we shall see later.
Duties, like wrongs, are of two kinds, being either moral or legal. A moral or natural duty is an act the opposite of which would be a moral or natural wrong. A legal duty is an act the opposite of which would be a legal wrong. It is an act recognised as a duty by the law, and treated as such in and for the purposes of the administration of justice by the state. These two classes are partly coincident and partly distinct. A duty may be moral but not legal, or legal but not moral, or both at once.
When the law recognises an act as a duty, it commonly enforces the performance of it, or punishes the disregard of it. But this sanction of legal force is in exceptional cases absent. A duty is legal because it is legally recognised, not necessarily because it is legally enforced or sanctioned. There are legal duties of imperfect obligation, as they are called, which will be considered by us at a later stage of our inquiry.