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a proposition that expresses permission or a requirement to perform or refrain from performing, under particular conditions, some act; the word “act” is understood to refer to some action or absence of action. These rules are called rules of permission and obligation, respectively; they are considered in a natural way to be elementary, or rules of the first rank, and are subsumed under the general term “injunction.” Complex rules are rules of the (n+ 1)th rank, obtained by applying injunctions to collections of rules of the nth or lesser rank in such a way that at least one of these rules must be of the nth rank. Ordinary grammatical rules are examples of rules of different, but not very high, ranks. A method is a system of rules of different ranks that includes rules designating the order in which other rules of the same system are introduced and rearranged.
Rules, whose systematic study is the object of deontic (normative) logic, are of importance in daily life and in all branches of science, particularly mathematics, logic, linguistics, ethics, jurisprudence, sociology, and political economy.