H. L. A. Hart (Herbert Lionel Adolphus Hart) (1907-1992) is widely regarded as the most important English-speaking legal philosopher of the twentieth century. He was Professor of Jurisprudence at Oxford University and is the author of The Concept of Law. Hart developed a sophisticated theory of legal positivism within the framework of analytic philosophy: He maintained that there is no necessary connection between law and morality; that for a legal system to be efficient there must be secondary legal rules to alter any primary legal rules of obligation; and that rules must be socially acceptable and recognized for them to be valid. Hart also made major contributions to political philosophy.