SIR ISAIAH BERLIN, a Latvian-born Oxford philosopher who died in 1997, may well have ranked among the greatest conversationalists who ever lived. According to Robert Darnton, a Princeton historian, Berlin's friends would “watch him as if he were a trapeze artist, soaring through every imaginable subject, spinning, flipping, hanging by his heels and without a touch of showmanship”. Darnton reckoned that Berlin's only match in relatively modern times might have been Denis Diderot, an 18th-century French Enlightenment philosopher. By one account Diderot's conversation was “enlivened by absolute sincerity, subtle without obscurity, varied in its forms, dazzling in its flights of imagination, fertile in ideas and in its capacity to inspire ideas in others. One let oneself drift along with it for hours at a time, as if one were gliding down a fresh and limpid river, whose banks were adorned with rich estates and beautiful houses.”