People who neither knew him well nor read him closely often found it clever to belittle Isaiah Berlin. To the scoffers, his reputation as a theorist of liberalism and historian of ideas was unmerited even at its peak in the 1950s and 1960s. Berlin, the theory ran, was a drawing-room philosopher, an establishment snob with an unoriginal mind, a confused and timid liberal overawed by status and power. He was foreign, he praised thinkers whom no one read - such as Vico and Herder - and he gabbled unintelligibly.

Envy aside, one obvious reason for such uncomprehending scorn is that Berlin was unusually hard to pigeonhole. He lived and breathed ideas but was not a technical philosopher. Politically, the left distrusted him, while the right appropriated and distorted his ideas. He was a bookish intellectual who operated socially at the top. "I've no feelings about people I've never seen," he writes to a friend in 1934, "unless they're very, very grand indeed." Yet he was unstintingly generous to all comers with his ideas and his time. He spent a career at Oxford without acquiring a trace of scholarly narrowness; history, philosophy and literature were as one subject for him. He might sound like a slippery, tender-minded wet. But even his artistic judgements reflected a strong moral outlook. Writing to a fellow philosopher, Stuart Hampshire, he reproves Marcel Proust and Henry James for hiding in "emotional neutrality" and loving "chiaroscuro for its own sake". To another friend, he describes his beloved Busch Quartet as inhabiting with Beethoven and Tolstoy "the same moral universe".

At the same time Berlin steeped himself in Oxford folkways, adopting a donnish tone of teasing silliness.To Hampshire in 1935, he describes the game of grading British monarchs: "George II wd have got a bad second, & Victoria would be viva'd for a first & not get it."

Away from academia, Berlin's correspondence has the wit and flow that friends prized in his conversation. He had a gift for the quick sketch and the wicked phrase. His friend David Cecil, an English literature don, has "a voice like a crate of hens carried across a field". E M Forster he calls "obviously skinless, very observant, capable of making one embarrassed about almost anything one says or does".

Berlin did not dispense praise on oath. Here he is, on best behaviour, writing about his first meeting with Aldous Huxley in a set of celebratory essays about the writer published long after, in 1965: "The picture I have attempted to draw may convey the notion that Huxley, for all his noble qualities, may . . . have been something of a bore or a preacher. But this was not so at all." And here is a more candid description of the same weekend at Victor Rothschild's house in Cambridge in 1936. Huxley, he tells his friend Mary Fisher, was "three-quarters dead and, I regret to have to repeat, boring".

Berlin wrote often to his many female friends, opening up to them shyly about his feelings. His love life in these years was hesitant and intermittent, though women fell readily in love with him. To the novelist Elizabeth Bowen, he describes an awkward break on a trip to Paris with a girl who thought he was going to propose to her: "Our meeting in the Zoo opposite the Python was exciting and tormenting in the extreme."

Berlin was labouring during these years on his book Karl Marx (1939). He writes little to correspondents about political thought, even less about pure philosophy. After the war, he explains, he is going to "evaporate" as a full-time philosopher. This was a half-truth. In many variants, he kept returning to three core ideas: that our deepest values, though real, conflict; that freedom from constraint by others (negative liberty) is more urgent or basic than freedom to flourish or "realise yourself" (positive liberty); and that liberalism fails if it cannot validate the universal need to belong. Though not himself closely analytical, Berlin had a dramatist's gift for personalising abstract ideas. His lifelong hero, Alexander Herzen, the exiled and homesick Russian foe of political tyranny and doctrinal simplicity, embodied all three of those central concerns. "Oh dear, Herzen," Berlin tells a friend in 1938. "There is no writer, & indeed no man I shd like to be like, & to write like, more."

Belonging was vital to Berlin. He considered his adopted England the best of countries, campaigned for a Jewish state in Palestine and, while detesting the Soviet Union, never lost a profound attachment to Russia, from where his parents, Mendel and Marie Berlin, had brought him as a boy of 11 in 1921. Many letters in this collection attest to the strength of these attachments. One is from the British embassy in wartime Washington, where Berlin was seconded to report on American politics. Asked to find who had leaked - and in effect killed - a proposal that Britain and the US declare the future of Palestine on hold until after the war, Berlin duly reported back in a letter of masterly duplicity. The informer was Berlin. Another, written to his parents in 1945 from Moscow, where he briefly worked at the British embassy, reads like an imaginary homecoming: "Need I describe the crunching snow . . ."

Henry Hardy's well-judged notes, often half a dozen to the page, are essential for keeping track in a forest of names. For fuller background, especially on the Washington years, many readers will want to refer to Michael Ignatieff's excellent life of Berlin, published in 1998, the year after his death. Questions arise about Hardy's selection. Berlin's weekly political reports from Washington were published in 1981, but almost all his other embassy letters from Washington are missing, something Hardy justifies by telling us they were too "opaque and esoteric" for general readers. Berlin had contacts at the highest levels in the American government and the Zionist movement. Are there any significant letters to such contacts, or were the "business" letters Hardy leaves out just thank-you notes and requests for stationery? He should have said.

Flourishing, Hardy tells us, is the first of what will be three volumes of Berlin's letters. Despite repetitions and longueurs, the first volume is full of riches. It evokes a vanished cultural world where philosophers read Latin and Greek, handfuls of privileged children went to university and nobody watched television. It gives us the inimitable voice of a generous, magnanimous intellect, although one already well known from many published volumes of essays. It is hard to imagine being so enthusiastic when the page numbers pass the 2,000 mark. Berlin had a modest, ironical temperament and a self-mocking tongue. Did he deserve this monument?

Edmund Fawcett has worked as a political correspondent in Washington, Paris and Berlin