By Maria Golovnina

LONDON, Feb 14 (Reuters) - American legal philosopher Ronald Dworkin, the man behind some of the most influential theories of law and morality in modern jurisprudence, died on Thursday. He was 81.

His family said in a statement that Dworkin died of leukaemia in London early in the morning. He is survived by his wife, two children and two grandchildren.

Dworkin, revered by both critics and friends for his sharp intellect and prolific writing, transformed the philosophy of law during his long career by making equality and human dignity the central pillars of English-language legal theory.

He is best known for his theory of law as integrity which states that propositions of law are true if they follow from the principles of fairness and justice, effectively encouraging judges to use moral considerations when deciding cases.

Dworkin overwhelmed his opponents with his ferocious debating skills, and was often unbeatable when defending his views. He was a committed Democrat and believed strongly in liberalism, equality and human rights.

"He was famous for making seemingly effortless presentations, composed in perfect sentences and paragraphs, but apparently delivered on the fly - almost always without notes," Legal Theory Blog wrote in a Thursday entry. "Dworkin was not one to give ground."

Dworkin was emeritus professor at University College London and a professor of law at New York University. He wrote on a variety of topics - often controversial - in his many books and articles, particularly for the New York Review of Books.

His work had tremendous impact on legal theory and he often waded into sensitive issues such as abortion, triggering intense debate in international philosophy and legal theory circles.

He was born in 1931 in the United States, studied at both Harvard and Oxford, and worked for a New York law firm before embarking on a fully academic career by becoming professor of law at Yale University.

In one of his best known books, "Law's Empire", Dworkin describes Hercules as an imaginary judge of superhuman intellect and reflects on the application of law in democracy. It received a prestigious award from the American Bar Association.

Dworkin was Professor of Jurisprudence at Oxford and Fellow of University College and had a joint appointment at Oxford and at NYU where he was a professor both in the Law School and the Philosophy Department.

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