Midway through The Third Man, Orson Welles’s penicillin-diluting racketeer Harry Lime defends his kiddie-killing activities with a big speech: “In Italy, for 30 years under the Borgias, they had warfare, terror, murder and bloodshed. But they produced Michelangelo, Leonardo da Vinci and the Renaissance. In Switzerland, they had brotherly love, they had 500 years of democracy and peace — and what did that produce? The cuckoo clock.” I don’t know whether A. L. Rowse ever saw The Third Man, but I do know that a few years after the movie was released he wrote a sentence whose shape and sentiment are very Lime-like: “Isaiah knows all these languages, is bilingual in English and Russian — knows everybody of importance in two continents. But what have all these talents, these advantages, produced? A couple of mice by way of books.”

Isaiah, of course, is Sir Isaiah Berlin, and in the mid-1950s, at the time of Rowse’s diary entry, he had indeed produced very little. In 1939, after years of painful gestation, Berlin’s Karl Marx: His Life And Environment had finally seen the light of day. Fourteen years later, The Hedgehog And The Fox — a book that would look an awful lot like a pamphlet had Lord Weidenfeld not thought to dignify it with hard covers — appeared. And that was that.

Sixty-odd years on, Rowse’s sniping makes a lot less sense. Though Berlin never did write a magnum opus — the long essay was always his métier  — the shelves strain under the weight of books that bear his name. The four volumes of Berlin’s Letters Henry Hardy has edited take up 12 inches or so. As for the lectures and articles and encyclopaedia entries Hardy has assembled for Princeton University’s complete edition of Berlin’s work, bound only in paperback they occupy another 14 or 15 inches.

Even allowing for repetitions — and Berlin’s stoutest defender would be hard-pressed to deny that his hero, having warmed to a theme, wasn’t averse to warming it up again — that’s no small achievement.

Still, if the line that Berlin produced little won’t hold, the line that he produced little of import clings on. He is most often thought of as a man who talked away much of what he might have written, thanks to those letters and to the self-mocking interviews he gave down the years, as well as to his love of dinner-table gossip and back-stabbing: Berlin more than once arranged for an enemy’s book to be given a kicking in the pages of the TLS; he was instrumental in preventing Rowse being elected warden of All Souls; and he blackballed Isaac Deutscher from a lectureship at the University of Sussex.

Lyons’s summaries of Berlin’s arguments are beautifully brief and to the point

Johnny Lyons is none too happy with Berlin’s flibbertigibbet reputation. The slander came about, he argues in The Philosophy Of Isaiah Berlin, not because of Berlin’s endless chit-chat, but because he himself didn’t think he’d been a real philosopher for all that long. Sometime towards the end of the Second World War Berlin forsook formal philosophy for the history of ideas. “I gradually came to the conclusion,” he said, “that I should prefer a field in which one could hope to know more at the end of one’s life than when one had begun.”

Nobody who has read Berlin could doubt that by his life’s end he knew a lot. Still, Lyons thinks Berlin is doing himself down. Just because Berlin called himself a historian of ideas, Lyons argues, it doesn’t follow that he had actually stopped doing philosophy. Rather, his journey through intellectual history was another way of being a philosopher. Berlin needed another way, Lyons says, because he didn’t want to waste his time on what he saw as the arid abstractions of the logical positivism and linguistic philosophy that were all the rage when he was starting out. Philosophy, Berlin maintained, shouldn’t be just footling talk about talk. It should be about life and lives. It should relate to — have purchase on — the real world.

Not that Berlin was any kind of materialist. He was fully on board, Lyons reminds us, with transcendental idealism — Kant’s still startling argument that while there really is a world external to the self, the self’s experience of that world is always and everywhere constrained by the limitations of its sensory faculties. Basically, we don’t see things as they are but as our eyes allow us to see them. It follows that whatever the scientistic hotheads claim, they can’t — and won’t ever be able to — explain everything. No matter what they uncover, the mysteries of life will remain. Hence what Lyons calls Berlin’s “unshakable conviction that a problem can be ‘perfectly meaningful without being strictly verifiable’.”

So far, so history of philosophy textbook. But Berlin complicated what Kant called his Copernican revolution by arguing that it had shaken the humanities as well as the sciences. Thanks to his reading of the Neapolitan thinker Giambattista Vico, Berlin came to see that our view of the past is no less susceptible to the Kantian turn. All our knowledge and thought, Berlin argued, is historicist. If it is not exactly engendered by our place in our culture (and our culture’s place in time) then it is nonetheless severely circumscribed by it. There is no such thing as a thought without a perspective, an argument without a backstory. It follows, Berlin held, that our first duty in getting a handle on a great thinker is to get to grips with the world he inhabited. It may be that what Rousseau had to say about the body politic is still relevant today. It may be, contrariwise, that Rousseau only seems relevant because we’re reading him with an eye to the present.

The task of the historian of ideas, then, is to imagine himself back into Rousseau’s age to work out how his arguments burgeoned from their originary moment. If this sounds difficult, says Lyons, that’s because it is. Nor, he admits, was Berlin’s attempt to meld the thought of Kant and Vico unproblematic. Nonetheless, the results were “philosophically fertile”.

Take Berlin’s pluralism. Following Machiavelli, Berlin came to see that certain values — freedom and equality, say — can both be true and in conflict with one another. Most people, that is, prize freedom — but they champion egalitarianism too. But while you can want them both, you can’t have them both (well, not unless you’re Ronald Dworkin, says Lyons). Taxation, for instance, might make a society a little more egalitarian — but in doing so it requires the surrender of some freedom on the part of those whose money is being taxed and redistributed. Whatever values we pursue, they cannot but come at a cost to other values, other people.

Worse, Berlin argued, ever since Plato the majority of philosophers had worked on the assumption that some kind of unified field theory of political philosophy that resolved everything for everyone forever was somewhere down the line. To believe that, he said, is to believe in just the same kind of fantasies that had mobilised the revolutions in France, Russia and Nazi Germany — fantasies that had led not to utopia but to hell on earth.

All this and more is covered by Lyons in his deeply impressive book. (It’s all the more impressive when you consider that Lyons abandoned academe for the commercial world 20 years ago, and has presumably done his donkey work in his spare time.) Broad in its field of references — the book quotes poets and novelists and journalists as well as a huge range of philosophers — it’s an exemplary work of humanistic thought.

Not that it is faultless. Certainly Lyons’s defence of Berlin’s defence of the idea of free will is less than convincing. Berlin’s Anselm-like argument that free will must exis t—because no matter how many times we’re told it doesn’t exist we refuse to let go of the idea — sounds mighty deterministic to me. And anyway, Lyons’s reference to Galen Strawson’s demolition job on free will (basically, we aren’t free to do other than we do because we are never free to be anyone other than ourselves) up-ends everything he’s just endorsed Berlin for.

But whatever the flaws in Berlin’s arguments, Lyons’s summaries of them — like those in Michael Ignatieff’s biography — are beautifully brief and to the point. Far briefer and more pointed than anything Berlin himself ever wrote. You don’t have to agree with Rowse, who quipped that Berlin was “unintelligible in several languages”, to acknowledge that he was prone to prolixity. Lyons’s great achievement is to condense and clarify Berlin’s thought and excavate the tragic insights that underlay this historian of ideas’ idea of history.