Yet John Gray's book rightly emphasises the crucial difference between the two men, overlooked by those, like Stuart Hampshire, who see Berlin as Humean all the way down. Hume, a typically Enlightenment thinker, thought that men and women were fundamentally the same everywhere; that their conduct obeyed basic laws and that science could discover these just as it could the laws of nature. Berlin has always quarrelled with this, arguing that the concepts and categories by which we order our world vary through time and space. You can only explain human conduct by getting inside a culture and its particular beliefs and values. Social science, searching for general, abstract laws, is bound to miss what is peculiar and specific.
In fact Berlin's rejection of the Enlightenment understanding of human nature lies at the very heart of his thought. It must have inspired his turn away from pure philosophy to history in the early 1940s, and is reflected in the way he has written the story of modern intellectual history as a battle between Enlightenment and counter-Enlightenment ideas. It also fired his interest in the history of nationalism and has given him a sympathy with its more moderate aspirations - a sympathy which must have set him apart from many of his left-liberal friends.
But although Berlin's hostility to the Enlightenment (and Marxist and Positivist) dream of turning history and society into an object of science is brilliantly articulated, it is not original. In fact he makes the case for it by tracing its development through long dead predecessors like Vico, Hamann, Herder, and Herzen. What is new about Berlin's thought is his argument that we are all faced with choices, in our lives, between incompatible and incommensurable values. Both ancient and modern philosophers have tended to assume there was a single right answer to moral and political questions - some formula that could resolve all ethical dilemmas - Berlin sees that life forces endless existential choices upon us. If we pursue community we must sacrifice autonomy, if we indulge in mercy we must sacrifice justice, if we aspire to knowledge we should be prepared to give up freedom. The belief behind so much Western political thought that an ideal society could at least be described, and perhaps even attained, is facile (and, as Marxism proved, dangerously so); living by some ideals is bound to exclude living by others.
Gray, a political theorist at Oxford who has had a long intellectual friendship with Berlin, is a devoted fan; he emphasises both the tragic quality of Berlin's thought and its "enormous subversiveness". But he also raises the question of how Berlin's "value pluralism" relates to his liberalism.
The common view, assumed to be Berlin's, is that if you accept that there are an array of conflicting lives worth living, then a liberal society can be justified as allowing people to choose for themselves the sort of life they want. But Gray ends his book arguing that this position simply does not hold: liberal societies admit a variety of ideals, but there are other ideals (piety, say, or solidarity) with which they are incompatible. And if, as Berlin's doctrine of value pluralism would suggest, we can't always rank the first ideals above the second, liberalism cannot be objectively superior to all forms of illiberalism. This position - that liberal freedom possesses "no universal claim on reason, foundation in human nature or privileged place in history" - is not one which Gray attributes to Berlin, but one, he argues, we should accept.
Gray has written a difficult book, but one which succeeds in making its subject seem profound and important. It should, once and for all, scotch the view that Berlin is just an especially imaginative reader, and help put him at the centre of moral and political thought where he undoubtedly belongs.
For all its qualities, though, this is in some ways a strangely inappropriate study. One of the memorable features of Berlin's work is its sensitivity to human peculiarity and variation. Berlin's essays, like Montaigne's, are catalogues of mutability. One of his favourite literary devices - the list - is almost worth an essay in itself. His historical writings show as much interest in the peculiar circumstances in which an idea emerges as the idea itself: his vivid eulogies to Einstein, Churchill, Chaim Weizmann and others (collected in Personal Impressions) display a similarly vivid eye for personal particularities. Berlin displays a romantic suspicion of abstract categories. He is an intellectual who loves concrete things.
Yet Gray pays hardly any attention to this aspect of Berlin's thought - how his writing doesn't only argue for an interpretative approach to history but exemplifies it, or how he does not merely put forward the rivalrous diversity of human ideals but also illustrates it. Unlike Berlin, John Gray seems much more at home with concepts than their embodiments. The result is a fine, urgent, exploration of Berlin's political ideas. The book makes unprecedented claims for the importance of his liberalism - "the most profoundly deliberated, and most powerfully defended in our, or perhaps in any time" - but it still does not quite do justice to the full richness of his work.