Every legal system contains mandatory laws, but there is no decisive linguistic marker that determines what they are. It is not necessary to use the term “obligation” or its term almost synonymous with “duty”. Mood is rarely found imperative. The Canadian Penal Code requires not condoning genocide: “Anyone who advocates or promotes genocide is guilty of a criminal offence and may be punished by imprisonment for a term not exceeding five years.” The English law on the sale of goods states: “If the seller sells goods in a transaction, there is an implied condition that the goods delivered under the contract are of merchantable quality.” The fact that these laws create obligations stems from the way the terms “offence” and “implied condition” operate in their respective jurisdictions, not from the language in which they are expressed. The basis of Rawls` theory of necessity becomes evident when we examine what it might mean for a just institution to “apply to us.” Simmons argues persuasively that an Institute for the Advancement of Philosophers cannot benefit us, however just it may be, and then demands that we pay its dues (Simmons 1979, 148). He therefore suggests that a normatively relevant sense of application requires accepting benefits – but that means transforming a natural mandatory account into a weakly voluntarist account like equity. (See below, § 5.3.) Jeremy Waldron diagnoses the power of such counter-examples as a consequence of the fact that the Institute, although functioning, is not something whose activities are required by the judiciary: they are optional, not necessary (Waldron 1993). This seems to be true; But if we then limit the realm of authority to necessity, we will again leave many legal obligations behind. Many of the activities of a legitimate government are voluntary. It must protect us from the state of nature, but the ambitions of the law are more ambitious than that. It also does things that are permitted but not necessary: it decrees residential zones, declares official languages, establishes national holidays, supports education and the arts, and creates honours.

And in the service of what is prescribed out of necessity, the law draws lines and sets standards that are themselves only permissible – an age of consent, an acceptable degree of imposition of risk, formalities for wills and marriages, etc. – what Thomas Aquinas called “determinations” of just requirements. The content of all this valuable and permissible state action is underdetermined by the theory of legitimacy and rests on considerations other than necessity. The necessary arguments therefore leave some – perhaps a lot – of valuable government action unsupported. Moreover, it is not clear what is necessary for the law to fulfil its socially necessary functions. Anscombe refers to the right to have what is needed for the role, but what is it? Hume considered it obvious that political society could not exist without “the precise obedience of the magistrate,” but it was certainly empiricism without facts. Everyone knows that a legal system can and does tolerate a certain degree of harmless disobedience and that this does not affect its functioning. A contractual obligation means that a person must comply with the instructions given or given due to the agreement, promise or oral/written contract between the persons involved. For example, Bernard has a contractual obligation to repay his money to Michael, provided that his rental agreement for the old apartment. An obligation may also refer to the letter or act by which a party testifies to the contract or agreement.

The obligations continue to exist even if a civil obligation is similar to an obligation containing a contractual penalty containing a condition of payment or performance. An invoice usually has no conditions or penalties, although the actions described may be required, distinguishing an invoice from an obligation. Another way of describing an obligation is an act that binds one party to another under a possible penalty for performing an act. It is interesting to note that this presentation assumes that one can say what the authority requires, whether or not the requirement is justified on the merits. Richard Friedman argues that “if there is no way of knowing whether a statement is authoritative other than by evaluating its content to see whether it merits acceptance as such, then the distinction between an authoritative statement and a rational advice or belief will have collapsed” (Friedman 1973, 132).