On producing his startlingly irreligious Letter on the Blind when he was thirty-six years old in 1749, Diderot was imprisoned for atheism. On his release after one hundred days, he flung himself into producing, with Jean le Rond d’Alembert, the thirty-five volume Encyclopedia, or a Systematic Dictionary of the Sciences, Arts, and Crafts, on its completion in 1772 regarded as the greatest intellectual achievement of the century. This compendium of secular knowledge, about everything from agriculture to zoology, was suffused with the scepticism of ‘reason’: ‘religion’ was grouped within the ‘Science of God’ on Diderot’s diagram of the tree of knowledge together with ‘superstition’ and ‘black magic’. The most radical article, by Diderot himself, was on ‘political authority’: ‘no man has received from nature the right to command other men … each individual of the same species has the right to enjoy [freedom] as soon as he is able to reason’.

Despite that monumental task – for which Diderot himself produced seven thousand entries (almost one-tenth of the total) – contemporaries sometimes felt that Diderot’s brilliance had never quite been realised in print. It was only later that they understood why.

A threat to his future liberty, made in a warning by the police after his imprisonment in 1749, forced him to withhold his most radical treatises from the public. His greatest individual works only started to surface after his death, notably his novels The Nun, Jacques the Fatalist, and above all Rameau’s Nephew. In these he excoriated religious intolerance, questioned custom and tradition as the basis of social order, and probed the subconscious and the meaning of love and sexual longing.

The most controversial novel, which he was at pains to hide, was Rameau’s Nephew. Written in 1761, it first appeared in German in a translation by Goethe in 1805, but the original French manuscript only turned up in a Paris bouquiniste on the Seine in 1890. This was Diderot’s most cynical, materialist analysis of morality and vice, laced with vitriol aimed at prominent contemporaries. Coincidentally, by the time the manuscript was found, Diderot’s reputation had recovered from the protracted rage of conservatives that he was a godless pedlar of smut and radicalism who had brought on the Revolution of 1789. By its centenary in 1889, he had been lionised by the secular Third Republic, and huge statues of him had been erected in his home town and Paris.

Portrait Of Denis Diderot by Dmitry Grigorievich Levitsky, 1773He wrote at least one-fifth of the most radical anti-slavery and anti-colonial treatise of the century, the Abbé Raynal’s Philosophical and Political History of the Two Indies (three editions, 1770–80). His Supplement to Bougainville’s Voyage, the result of his fascination with the explorer’s stay in Tahiti in 1768, was similarly prescient on the impact of colonialism and sexual predation. But Diderot also left a mass of other material ranging from revolutionary art criticism to a reformist political treatise for his generous patron Catherine the Great, which she shunned. His creative energy was astonishing. Not until 1948 was the final vast cache of his writings located in a château in Normandy.

The brilliance and breadth of Diderot’s intellect not only inspired novelists such as Balzac and Zola, but also influenced Marx’s understanding of class. Diderot was a frequent adulterer and writer on sexuality, and Freud credited him with the earliest understandings of childhood sexual desires. The Nun told a harrowing tale of the sexual and mental abuse a nun suffered when she tried to leave her order. But he also left volumes of intimate and precious letters to his greatest love, Louise-Henriette Volland (he called her ‘Sophie’ to refer to her wisdom). Over thirty years (1755–84), he wrote her 553 letters, of which 179 survive. None of hers do.

His intellectual range was breath-taking, coruscating the elaborate deceits of autocracy and ‘superstition’ (his word for religious belief) but also warning, after 1776, of the dangers in the new and republican United States of America of ‘the affluence of gold that brings with it the corruption of morals and the scorn of laws’. He warned of the dangers of autocracy in large nations.

Andrew S. Curran is a professor of history at Wesleyan University in Connecticut and the author of two other remarkable and prize-winning books, one on the mental constructs of physical monstrosity in Diderot’s writings, the other on ‘scientific’ constructions of blackness and race at the time of the Enlightenment. This latest book is equally fine, a wonderfully readable as well as expert introduction to Diderot’s life and work.

At times it is rather breathlessly enthusiastic, assuring us that Diderot ‘intentionally’ kept his writings secret so he could communicate with us rather than his contemporaries. Perhaps his decision to keep his most radical writings secret stemmed instead from a judicious desire for self-preservation from the police or his wish to remain close to an autocrat like Catherine, with whom he stayed in St Petersburg in 1773–74, to whom he acted as an art broker as well as an advisor on reform politics, and to whom he left his library and many unpublished works. One need not accept Curran’s implication that he was somehow speaking beyond the grave to liberals in Trump’s America. It is difficult, however, to disagree with the author that this extraordinary polymath was ‘the most creative and noteworthy thinker of his era’ and a ‘genius’: ‘his joyful and dogged quest for truth makes him the most compelling eighteenth-century advocate of the art of thinking freely’.