Sade was imprisoned at the instigation of his mother-in-law under a lettre de cachet, the notorious instrument of the ancien regime, whereby the king's word was sufficient to lock up anyone, indefinitely and without trial. In July 1789 he was in the Bastille and caused a riot outside the prison by claiming that the prisoners were being killed; he was quickly moved to Charenton lunatic asylum. Days later the Bastille was stormed; so Sade has a claim to have started the French Revolution. 

Briefly released during the Terror, "Citizen Sade" was elected to the National Convention as an acclaimed orator, but his suicidally brave criticism of Robespierre saw him locked up again for the crime of "moderatism". Released again after Robespierre was guillotined, he later fell foul of Napoleon, who banged him up at Charenton for his authorship of Justine and Juliette. 

Theodor Adorno, one of many academics who has given serious attention to Sade, thought Juliette embodied the most perniciously amoral elements of Enlightenment thought. As his imprisonment lengthened, Sade became obsessed with numerology as a superstitious antidote to his confinement, while railing in his writings against the despotism and tyranny into which post-Revolutionary France had descended. Sade defied the dominant thinker of his day, Jean-Jacques Rousseau, by denouncing the "moronic" glorification of man's state of nature, a doctrine which in his view had produced slaughter and cruelty beyond anything conjured in his own works. Concealed by the shock tactics and the satire (to suggest that Sade believed in slaughtering children is as absurd as to suggest that the Swift of A Modest Proposal believed in eating them), there is an inconsistent yet serious thinker, sometimes fiendishly funny, peeking from beneath the bedclothes. 

Is Sade the source of a depressingly debased sexual culture in which women are encouraged to objectify themselves before an essentially misogynist definition of pleasure? Anyone who believes so might be surprised to read his advice to Eugénie, the ingénue of Philosophy in the Boudoir: "You are the only person in the world who has the right to enjoy your body." Much of his writing is surprisingly modern: it is absurd to penalise sodomy, he argues, while young women must dismiss "the chimera of virtue" if they are to become true citizens rather than goods bartered on the marriage market. 

But sexy it is not. It is the English, we should recall, who as a nation went wild for flagellation. The decadent poet Swinburne, himself an enthusiast of le vice anglais, invited his friends, public schoolboys all, for a reading of Sodom with added whipping. But the gathering collapsed into hilarity when the forbidden work was discovered to be a bit, well, tame. That, perhaps, is the only thing that Mistress James and the Divine Marquis really have in common.