From 1952 on he delivered the undergraduate lectures that turned into The Concept of Law (1961). He held seminars with Tony Honre on causation, leading to their joint work Causation in the Law (1959).
His visit to Harvard in 1956-7 led to his Holmes lecture on 'Positivism and the Separation of Law and Morals' (1958) and a famous controversy with Lon Fuller. Returning to the UK he engaged in an equally famous debate with Patrick (later Lord) Devlin on the limits within which the criminal law should try to enforce morality.
Hart published two books on the subject, Law, Liberty and Morality (1963) and The Morality of the Criminal Law (1965). A wider interest in criminal law, stimulated by Rupert (later Professor Sir Rupert) Cross was signalled by his 'Prolegomenon to the Principles of Punishment' (1959).
Hart resigned his chair in 1969, to be succeeded by Ronald Dworkin , a severe critic of his legal philosophy. He now devoted himself mainly to the study of Bentham, whom, along with Kelsen, he regarded as the most important legal philosopher of modern times.
From 1973 to 1978 he was Principal of Brasenose College . In his last years he was much concerned to find a convincing reply to Dworkin's criticisms of his version of legal positivism.
He was the most widely read British legal philosopher of the twentieth century. He passed away in 1992.