Isaiah Berlin (1909-1997) lived in ideas. Not as a solitary resident in an autonomous republic of abstractions. His was a crowded, generously ventilated apartment in downtown History. He listened to ancient voices. He looked out for the hidden passages of the Zeitgeist. It was a permanent obsession with the grammar of liberty.

Liberty for Berlin was not an impersonal, singular concept. It was a `personal' rejoinder as well as a moral explanation, resonant with seemingly conflicting ideas: ``Men choose between ultimate values; they choose as they do because their life and thought are determined by fundamental moral categories and concepts that are, at any rate over large stretches of time and space, a part of their being and thought and sense of their own identity; part of what makes them human.''

It is rooted in pluralism. It not only repudiates the rationalist ideals of authoritarian structures but celebrates the humane ideal. There are two kinds of liberty. For Berlin, `negative liberty' seemed truer and more humane, which allowed the individual a freedom not defined by the state. Positive liberty is conditioned by the state. ``To assume that all values can be graded on one scale, so that it is a mere matter of inspection to determine the highest, seems to me to falsify our knowledge that men are free agents, to represent moral decision as an operation which a slide-rule could, in principle, perform,'' he wrote in the famous essay `Two Concepts of Liberty'.

That was Philosopher Berlin, concerned with concepts and categories. And the essay appeared at a time when the certainties of Marxism were exiling a vast section of humanity to a deceptive idyll. Berlin's rejection of the received truth, his `theoretical' dissent, was steeped in historical memory. Born in Riga, capital of Latvia, he was a Jewish child who had later experienced the first stirrings of the Russian Revolution in Petrograd. He came to England at the age of eleven. First as a student, and later as a teacher, Berlin spent most of his life at Oxford. His life was witness to the tectonic shifts of this century: the Bolshevik Revolution and the subsequent Soviet tyranny, the Holocaust, the Second World War. His words are a personal testament that echoes the passions of history.

And it is as a historian of ideas that Berlin will be remembered. Ideas as a dramatic passage inhabited by rare minds. It is the genius of men that provides the subtext to changes, both epochal and conceptual. Machiavelli, Herzen, Akhmatova, Pasternak, Churchill, Roosevelt -- the fascination of Berlin has brought out some intimate biographies of ideas. As Noel Annan writes in a forward to the recently published anthology of Berlin, The Proper Study of Mankind (Chatto & Windus, 667 pp; £ 25), ``Like Hamlet, he stands amazed at `What a piece of work is a man'; unlike Hamlet he delights in man.''

Watch that delight in his most celebrated essay on Tolstoy, `The Hedgehog and the Fox'. He begins with a line from the Greek poet Archilochus -- ``the fox knows many things, but the hedgehog knows one big thing''. Plato, Hegel, Dostoevsky, Nietzsche, Ibsen and Proust are, for instance, hedgehogs. Tolstoy, a natural fox, tried to become a hedgehog. The result: ``Tolstoy's sense of reality was until the end too devastating to be compatible with any moral ideal which he was able to construct out of the fragments into which his intellect shivered the world, and he dedicated all of his vast strength of mind and will to the lifelong denial of this fact.''

Equally delightful is his essay on Machiavelli, who is more than the apostle of `Machiavellianism'. He juxtaposed the two incompatible systems of morality: Christian ethics and Roman Republicanism. ``He helped to cause men to become aware of the necessity of having to make agonising choices between incompatible alternatives in public and private use.'' Berlin could not live without the company of such divided minds (most of them were understandably Russian).

In a word of pure Machiavellianism, of moral uncertainties, Isiah Berlin, philosopher, elucidator, conversationalist, bon vivant, lived a fulsome life that has so luminously enhanced the Kantian wisdom: ``Out of the crooked timber of humanity no straight thing was ever made.''