Part of the problem lay with the nature of Berlin’s early existence. Despite the undoubted significance of the Washington interlude (that said, remember everyone else was at war), his was a don’s life. Quintin Hogg remembered the young Berlin as “a terribly typical don”. That cannot have been true. His principal preoccupations until the mid-1950s — doing and teaching philosophy — enabled him to live and work in a world where the world of mouth largely still largely prevailed over the realm of writing, both formally and informally. This is not to criticise. Far from it. Still, it is worth observing just how few letters there are in these early volumes to his fellow Oxford philosophers J.L. Austin, A.J. Ayer and even Gilbert Ryle. Indeed, it is significant that there are not many references to them, whether about their thoughts, or even  their writings, in these pages at all.

That, circumstantial, self-censorship receded considerably in later life. Success brought broader engagement with Berlin’s ideas. The growth of attendant interest finally stimulated him, if not to write more books, then at least to write to many of his intellectual peers and contemporaries about what he was doing and why he was doing it. It also enabled — perhaps it also compelled — him to express himself more formally about worlds beyond the Oxford cloister. The rest of us are the long-term beneficiaries of this, subtle but significant, alteration in his way of life. The intellectual qualities of volumes three and four, especially, are very much higher. So too is their intellectual generosity. Indeed, much the most attractive aspect of the whole correspondence, much more noticeable in Affirming than in any preceding volume, is the repeated demonstration of Berlin’s willingness to explain himself, often at considerable length to young and obscure scholars who sought his instruction and, one suspects, also his encouragement. Both were invariably forthcoming, and often quite touchingly expressed.

To be sure, both this quality, and indeed that generosity, were often double-edged. The inadvertent great man sometimes slipped rather too easily into a grand authority. Read his letter to Edward Adeane on who should (or should not) be considered for the next Order of Merit, c.1993, and gasp. Even the occasional “to the extent that I can judge” might have helped. The much-fêted professor more seldom doubted his own importance, indeed originality. He usually made these points modestly, but he meant them all the same. The idea of a so-called “incommensurability of values” was, he insisted, “not to be found in most of the political philosophers of whatever age”. Much rests on the meaning of the word “most” in that context.

True, Berlin often described himself as “overrated”. However, he no less frequently depicted others — usually his intellectual peers — in considerably less flattering a light. His judgment on Michael Oakeshott (“a feeble heir to Montaigne . . . with not one idea, or even formulation to call his own”) was uttered in the very year of the publication of On Human Conduct. His attitude to Hannah Arendt was little short of demented, and rooted in more of a personal grudge than he was ever prepared to admit. Leo Strauss was alternatively ignored or dismissed. Compare Berlin with Raymond Aron in this respect. Then observe how rarely — how astonishingly rarely — the name of Aron appears throughout the whole of this correspondence. Perhaps Berlin implicitly understood that any serious comparison between the two of them might not turn out to his advantage.