I first met him in 1943 when he was writing weekly political reports for the British Embassy in Washington. His acute insight into American affairs made his dispatches to London required reading at the highest level. On a famous occasion Winston Churchill, hearing that Berlin was in London, summoned him to luncheon at 10 Downing Street and asked him penetrating questions about American politics. Disappointed by the answers, he said later, ““Berlin writes better than he talks.’’ In fact, he had invited the wrong Berlin–Irving, not Isaiah.

The man I met in 1943 was 34 years old, black-haired, balding, with alert, snapping eyes and a smile of delighted discovery. He seemed then a man of indeterminate middle age; in his 80s he still seemed a man of indeterminate middle age; he changed in appearance less than anyone I have ever known. I still see him that first night in Washington, sitting on a sofa, talking at immense speed in a torrent of words, at first hard to understand until you surrendered to the cascade and let the meaning pour into you. The sentences were intricate and erudite but fascinating, filled with wit, warmth and humanity. He never pulled intellectual rank. He had enormous generosity of spirit, an unparalleled sense of fun and unquenchable delight in the varieties of human experience. And he had the marvelous quality of intensifying life so that one perceived more and thought more and understood more.

He had begun as an analytical philosopher, but after the war he abandoned philosophy and turned to the history of ideas. ““Philosophical problems,’’ he said, ““did not keep me awake at night . . . I realized that I wanted to know more at the end of my life than at the beginning.’’ Ideas were the great educators. They were the entry into the mysteries of human existence. Ideas were the triggers of action and the key to humanity’s hopes, visions, follies, illusions, terrors and triumphs.

His central thought was pluralism–a much abused and not very inspiring word, but one that, as he employed it, celebrated the diversity of life and the irreducible collision of values. Many values are perfectly compatible, but other values are not. Absolute freedom, for example, is incompatible with absolute equality: ““If you choose one value,’’ Berlin said, ““you must sacrifice another.’’ The tragedy of choice becomes an argument for tolerance, compromise, for trade-offs. Franklin Roosevelt was his beau ideal of the man who could lead into the good society, and he remained an unabashed New Dealer to the end of his life.

The great human delusion, Berlin thought, is monism–the proposition that there is a single, final solution, an ultimate and overarching truth, that harmonizes all values and that justifies the sacrifice of living beings to grand abstractions–““the victimization of the present for the sake of an unknowable future.’’ The pursuit of perfect harmony is ““a fallacy, and sometimes a fatal one’’–hence his passionate opposition to communism and fascism. At the same time he recognized the temptation of monism as an abiding human trait. His most powerful essays enter empathetically into the mind of irrationalism. No one has written more perceptively about the critics of democracy and freedom and their offer of certitude in an ambiguous world.

Two other delusions, he believed, bedevil humankind–the delusion of relativism, that all values are more or less equally valid; and the delusion of determinism, that the individual makes no difference to the course of history. His superb essays on Churchill and Roosevelt make the point. You must stand unflinchingly as they did, Berlin said, for what you believe.

In a short space, one can only suggest the exciting richness of Berlin’s thought and direct readers to the recent anthology of essays, ““The Proper Study of Mankind.’’ His polarities illuminate–the Enlightenment vs. Romanticism, negative vs. positive liberty, the Hedgehog, who knows one big thing, vs. the Fox, who knows many things. He was a beacon of wisdom and humanity in what he called the ““most terrible century in western history.’’ And for those lucky enough to have known him–and many did, because he was so kind and approachable and unassuming and had an unexcelled talent for friendship–the memory of this man of surpassing brilliance and singular sweetness is imperishable.