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HLA Hart was a successful practicing barrister, who turned to a career in academic philosophy after serving with the security services during WWII. His book The Concept of Law is probably the most influential textbook on jurisprudence ever published, and has been translated into 26 languages. In this book, Hart set out to define what 'law' is. His thesis was a form of Legal positivism, in that he saw law as created by people for their own benefit, not something derived from morality, nature, or reason. However, Hart rejected the traditional view of Austin that laws were nothing more than commands backed up by threats. Indeed, he argued that trying to reduce the technical intricacies of, say, contract law to commands backed by threats was simply an absurdity.

In addition, Austin's view failed to account for a number of observed features of sophisticated legal systems. For example, Austin's command theory cannot explain why legal systems have continuity across changes in government. Moreover, the unrestricted power enjoyed by Austin's sovereign does not exist in practice in any stable legal system. Hart argued that attempts to define what 'law' was inevitably failed because people were attempting to define law in terms of things that were no better understood than 'law' itself. To define one thing by means of its similarities to, and differences from, other things is only helpful if we first understand those other things. Hart argued that we should define law by means of sentences in which the word was validly used, and not try to reduce it to synonyms.

Hart went on to claim that we can recognize a law by the status it occupies in society. It is not sovereign command that makes people obey the law, it is the expectation of society as a whole. He gives as an analogy the game of chess. When two people play chess, they do not require threats to ensure they play by the rules: the players simply expect it of one another. Similarly, we expect that other people will conform to the laws of our society. Laws are, therefore, normative rules. A person is obeying a law when he uses the law to guide his actions, and expects other people to do likewise. For law to be a system of rules, there must be a straightforward test to determine whether something is, or is not, a rule. Hart was adamant that, in any given society, there could be found such a test, which he called a 'rule of recognition'.

The rule of recognition has features analagous to the Grundnorm of Kelsen, but Kelsen saw the grundnorm as validating, not classifying other rules. Although Hart rejects the Natural law notion that law must be based on morality, he concedes that any legal system will contain an irreducible minimum of rules that appear to be derived from natural law. This, he argues, is inevitable, given that legal systems exist to safeguard the survival of societies of human beings. It is a matter of plain observation, for example, that people are vulnerable to bodily attack and have limited altruism. As a result, a successful legal system must protect peoples' bodily integrity and allow for the sharing of resources. This is not because law is based on morality, but because it has to serve human beings with particular characteristics.

Hart's theory has been extensively and minutely criticised by Dworkin, one the basis - among other things - that law is not rule-based, it is principle-based; Dworkin attacks the coherence of the rule of recognition, which is in many ways a foundational principle of Hart's thesis. See also Hart's theory of law, Dworkin, Lon Fuller.