Sir Isaiah Berlin, OM (6 June 1909 – 5 November 1997) was a philosopher and historian of ideas, regarded as one of the leading liberal thinkers of the twentieth century. He excelled as an essayist, lecturer and conversationalist; and as a brilliant speaker who delivered, rapidly and spontaneously, richly allusive and coherently structured material, whether for a lecture series at Oxford University or as a broadcaster on the BBC Third Programme, usually without a script. Many of his essays and lectures were later collected in book form.

Born in Riga, now capital of Latvia, then part of the Russian Empire, he was the first person of Jewish descent to be elected to a prize fellowship at All Souls College, Oxford. From 1957 to 1967, he was Chichele Professor of Social and Political Theory at the University of Oxford. He was president of the Aristotelian Society from 1963 to 1964. In 1966, he helped to found Wolfson College, Oxford, and became its first President. He was knighted in 1957, and was awarded the Order of Merit in 1971. He was President of the British Academy from 1974 to 1978. He also received the 1979 Jerusalem Prize for his writings on individual freedom.

Berlin's work on liberal theory has had a lasting influence. His 1958 inaugural lecture, "Two Concepts of Liberty", famous for its distinction between positive and negative liberty, has informed much of the debate since then on the relationship between liberty and other values.

Two Concepts of Liberty was the inaugural lecture delivered by Isaiah Berlin before the University of Oxford on October 31, 1958. It was subsequently published as a 57-page pamphlet by Oxford at the Clarendon Press. It also appears in the collection of Berlin's papers entitled Four Essays on Liberty (1969) and was more recently reissued in a collection entitled simply Liberty (2002).

Berlin distinguished between two forms or concepts of liberty – negative liberty and positive liberty – and argued that the latter concept has often been used to cover up abuse, leading to the curtailment of people's negative liberties "for their own good".

Berlin believed that positive liberty nearly always gave rise to the abuse of power. For when a political leadership believes that they hold the philosophical key to a better future, this sublime end can be used to justify drastic and brutal means. Berlin saw the vanguard elite of the Soviet Union as a prime example of the dangers of 'positive liberty' and the concept can be seen as especially salient during the Cold War, where revolutionary sentiment was rife.

Berlin believed that a more precautious principle was needed, and that was 'negative liberty', where individuals are protected against radical or revolutionary messages, and thus have little grand or existential freedom but are granted the more 'internal' liberty to pursue recreational and consumer interests.