He subsequently became Principal of Brasenose College, Oxford. Hart strongly influenced the application of methods in his version of Anglo-American positive law to jurisprudence and the philosophy of law in the English-speaking world.
Influenced by John Austin, Ludwig Wittgenstein and Hans Kelsen, Hart brought the tools of analytic, and especially linguistic, philosophy to bear on the central problems of legal theory.
He received a Harmsworth Scholarship to the Middle Temple and also wrote literary journalism for the periodical John O'London's Weekly.
During World War II, Hart worked with MI5, a division of British military intelligence concerned with unearthing spies who had penetrated Britain, where he renewed Oxford friendships including working with the philosophers Gilbert Ryle and Stuart Hampshire.
Hart did not return to his legal practice after the War, preferring instead to accept the offer of a teaching fellowship (in philosophy, not Law) at New College, Oxford. The two jointly taught from 1948 a seminar on 'Legal and Moral Responsibility'.
Among Hart's publications at this time were the essays 'A Logician's Fairytale', 'Is There Knowledge by Acquaintance?He worked closely with Dick White, later head of MI5 and then of MI6.Hart worked at Bletchley Park and was a colleague of the mathematician and codebreaker Alan Turing.; 18 July 1907 – 19 December 1992), usually cited as H. He was Professor of Jurisprudence at Oxford University and the Principal of Brasenose College, Oxford. Hart, was a British legal philosopher, and a major figure in political and legal philosophy.The description appears in her book The Spiral Staircase. Many of Hart's former students have become important legal, moral, and political philosophers, including Brian Barry, Ronald Dworkin, John Finnis, John Gardner, Kent Greenawalt, Peter Hacker, David Hodgson, Neil Mac Cormick, Joseph Raz, Chin Liew Ten and William Twining.Hart retired from the Chair of Jurisprudence in 1969 and was succeeded by Ronald Dworkin. Hart also had a strong influence on the young John Rawls in the 1950s, when Rawls was a visiting scholar at Oxford shortly after finishing his Ph D.The Harts had four children, including, late in life, a son who was disabled, the umbilical cord wrapped around his neck having deprived his brain of oxygen.The boy was, despite his handicap, capable of remarkable observations on occasion.It was in the summer of that year that he began writing his most famous book, The Concept of Law, though it was not published until 1961.In the interim, he published another major work, Causation in the Law (with Tony Honoré) (1959).