Herbert Lionel Adolphus Hart was a British-educated legal philosopher whose writings adhere to the philosophical category of legal positivism. The rule of recognition is one of the central concepts governing his philosophy.
Hart's Rule of Recognition underlies all other possible rules that may be made in a legal system. It is a collective agreement by the adjudicators of law to utilise a certain set of standards and criteria to govern. By definition, a decision or rule cannot be legally valid if it does not adhere to the Rule of Recognition.
Hart's Rule of Recognition is paramount to what Hart calls "primary" and "secondary" rules of law. Primary rules are defined as directives for behaviour that organise and govern society, while secondary rules govern how the primary rules are enforced, and what shape they take. For example, portions of United States law that detail how to amend existing laws are what Hart would call secondary rules.
Hart's concept of law, positing the Rule of Recognition, separates his approach from other legal positivists, like John Austin and Ronald Dworkin. These ideas remain highly influential to students of legal philosophy.