Sir John W. Salmond, Jurisprudence (1913)
“ 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. ”
