Denis Diderot, at Catherine the Great’s insistent invitation, spent the autumn and winter of 1773–4 in St Petersburg. It was the worst time of year for an ageing philosopher, underdressed, plagued by catarrh and colic and determined not to be the cause of unnecessary expenditure or trouble. Catherine had already ‘bought’ him by purchasing his library (which she would only gain possession of upon his death), by paying him to be her librarian and by offering to publish in Riga the unfinished Encyclopédie, his life’s work, uncensored.

As Diderot himself cannily remarked in Rameau’s Nephew, since Roman times rulers had hosted at their courts an appointed fool but ‘at no time was there an appointed sage’. Catherine sought to set a different example. Her successors, however, once again dispensed with court sages: it was not until Stalin came to power that a Russian ruler once more tried to do business with philosophers. Both Catherine and Stalin, more often than not, eventually made their sages look like fools. H G Wells, George Bernard Shaw and Romain Rolland were crippled by awe in Stalin’s presence. The bolder collocutors – Henri Barbusse, Maxim Gorky – died in suspicious circumstances. Catherine, however, appeared to be a genuine seeker of inspiration as well as flattery, and her French was as faultless as her native German (at her court, Russian was reserved for addressing servants, soldiers and God, or for composing bad verse). Catherine’s extensive correspondence with Voltaire, never sullied by a physical encounter, was a decades-long mutual massage that did her no harm and only slightly blotched Voltaire’s reputation. (Voltaire had already compromised himself by writing an adulatory history of Peter the Great, relying largely on material supplied by the Empress Elizaveta.) Some French philosophers, notably Jean le Rond d’Alembert and, surprisingly, Jean-Jacques Rousseau, spurned Catherine’s blandishments.

Diderot, unlike others, succumbed to what he felt was his duty. Once in St Petersburg he gained access to Catherine for several hours almost every afternoon, speaking to her with a shocking lack of decorum, alternating flattery with outright criticism, even pinching her thigh until (she alleged) it was black and blue as he urged her to abolish serfdom, to put herself and her successors under the rule of law and even to move the capital city back to Moscow. The St Petersburg weather and water took revenge on Catherine’s behalf: bedridden with chills and dysentery, Diderot was eventually forced to cut back on his interviews; he returned to France looking ten years older. Although Diderot did not stop buying works of art for Catherine’s Hermitage and Catherine continued to subsidise him until his dying days (she bought him a palatial ground-floor apartment in Paris to spare him the ordeal of climbing stairs), he was merciless in his critique of her grandiosely written (to be precise, half-plagiarised) political programme, the Nakaz, and the relationship soured. Diderot and Catherine realised that both philosophers and autocrats were irredeemably intransigent, the former living outside political reality, the latter confined by it.

Catherine, in her letters and in reports of her conversations (some reworked by herself), showed, as did Stalin later, a formidable range of reading and a mastery of political philosophy equalled only by her ill-disguised contempt for it. Diderot, a man of exemplary honesty, found it hard to cope with her dual nature. He had no trouble, unlike Voltaire, in dismissing Frederick the Great of Prussia as a monster, and on his way to St Petersburg deliberately made a detour to avoid an unpleasant encounter with him in Berlin. But when confronted with the dark side of Catherine, he was weak enough to excuse her notorious complicity in the murder of her husband, Emperor Peter III: ‘not all truths are good to speak,’ he explained.

After abandoning her attempts to reform Russia’s legal and educational systems, Catherine turned instead to ruthless expansionism, taking part in the divisions of Poland while also detaching the Crimean Khanate from Turkey, subjecting the peninsula to ethnic cleansing and imposing a puppet khan on the territory. She also planned the incorporation into Russia of Georgia, first allowing it to be ravaged by Persian warlords. Diderot was so obsessed with the need to defeat the Ottoman Empire that he ignored Catherine’s destruction, well noted by French travellers, of the Crimean Khanate, which in its tolerance, literacy levels and civic amenities was equal to most western European states and the independence of which the Russians had recognised in 1774. The only reproach that Diderot ventured was that military might was no help in creating a more just state. Diderot praised Catherine’s ‘humanity’ in abolishing torture but ignored the quartering of the Cossack rebel Pugachev, her order to kill the young Ioann Antonovich (the imprisoned heir to the Romanov throne) and her kidnapping and incarceration of the wretched would-be usurper Princess Tarakanova.

Catherine’s achievements and failings have been exhaustively examined by British historians, notably Isabel de Madariaga. Robert Zaretsky, to judge by his bibliography and gross misspellings of Russian names, does not understand a word of Russian and has used other historians as his sources. Like Diderot, he glosses over the worst that Catherine did. He gets quite a few details wrong: Cossacks descend more from runaway serfs than from Tatars; Moscow may be no warmer than St Petersburg, but Diderot was right in thinking that St Petersburg’s humidity and winds made it a more hostile environment (the city’s cemeteries are full of the graves of foreign artists, philosophers and businessmen who have underestimated its climate).

This book is particularly good, though, in its advocacy of Diderot as a thinker and imaginative writer. Diderot’s misfortune was that he preferred to see the world in his imagination, content, like Pascal, not to leave his study, and his best writings were published posthumously (notably Rameau’s Nephew and Jacques the Fatalist) or took the form of contributions to larger works, notably the Encyclopédie, for which he wrote many articles and rewrote thousands of others. The result is that Diderot still does not share the fame of Voltaire and Rousseau. He lacked Voltaire’s creative polish. He never distinguished himself on the stage and his verse is insignificant. Even his infamous lines ‘His hands would plait the guts of the last priest/For want of a rope to strangle the last king’, which so inspired Alexander Pushkin, merely versified the curt utterance of the atheist abbé Jean Meslier.

Nor did Diderot share Rousseau’s Romantic genius. But, as Zaretsky demonstrates convincingly, he was the French equivalent of Laurence Sterne, as well as a Samuel Beckett before his time and an effervescent source of witty paradoxes. Nothing Voltaire or Rousseau wrote has the daring of Rameau’s Nephew and no one individual has ever had the endurance needed to put together, single-handedly at times, a compendium of all human knowledge. The Encyclopédie is still in print and much of the information there is not to be found in any modern encyclopedia: anyone repairing an ox cart for a rural museum, designing costumes for an 18th-century drama or trying to make sense of a theological dispute may find Diderot’s entries in the Encyclopédie their sole resort. Moreover, it is saturated with a nihilistic wit conspicuously absent from Wikipedia.

The author is clearly as sure-footed an expert on the French Enlightenment as he is a shaky novice when it comes to Russia’s 18th-century Europeanisation. The book suffers from occasional linguistic awkwardness: translations from French are sometimes too literal – ‘losing time’ for ‘wasting time’, for instance. Zaretsky’s English can be brash: words like ‘moniker’ jar in an 18th-century context. These faults are insignificant, however, compared with his metaphorical achievement of what François Hollande promised but failed to do in practice during his presidency, which was to move Diderot to his rightful place in the French Panthéon.