Voltaire mocked the priests and kept his God. Diderot went to prison in 1749 for a pamphlet about blind men, and afterwards wrote his boldest books to leave in a drawer, unpublished. Enlightened Europe had learned how to doubt in private and how to hedge in print, and the live question was no longer whether the churches could be embarrassed: it was whether anything held without them — whether a universe of nothing but matter left a person any ground to stand on when deciding how to live. Nobody had answered that in public until a rich German baron in Paris answered it in two volumes, over a dead man's name.
Système de la nature appeared in 1770 under the name of Jean-Baptiste de Mirabaud, a former secretary of the Académie française ten years in his grave. Its argument runs without a gap and without a God. Nature is matter in motion and nothing else; mind is what a living brain does, so nothing survives the body; every event follows necessarily from the one before it, which makes free will a feeling rather than a fact; religion begins in fear of causes nobody can see. And morality survives all of it intact, because what people actually want is to be happy, and no one is happy among others he has wronged.
That came out of a decade of scientific translation rather than out of any taste for provocation. Between 1752 and 1765 d'Holbach wrote some four hundred signed articles for Diderot's Encyclopédie, most of them on chemistry, mineralogy and metallurgy, and he spent the same decade putting the mineralogists of Germany and Sweden into French — Wallerius in 1753, Henckel in 1756, Lehmann in 1759. He had gone through the mineral kingdom substance by substance. Matter, handled that closely for that long, kept turning out to be enough.
A fortune inherited from his uncle in 1753 bought a table on the rue Royale that ran twice a week for thirty years, and through it passed Hume, Adam Smith, the novelist Sterne, the historian Gibbon, Franklin and Beccaria, the Milanese jurist. Rousseau came, quarrelled, and left the circle with a contemptuous name attached to it that has stuck ever since. Diderot's hand is somewhere in the books; Naigeon, his disciple and collaborator, carried the manuscripts to Amsterdam, where Marc-Michel Rey, the leading publisher of the philosophes, printed them for smuggling back across the border.
The Parlement of Paris, the kingdom's chief court, condemned Système de la nature to be burned on 18 August 1770. Voltaire, who wanted no part of atheism, attacked it in his Dictionnaire philosophique; Frederick the Great, who wanted no part of it either, wrote out a refutation of his own; the young Goethe found it so bleak that it turned him against French philosophy for good. In those same years the courts branded booksellers' agents and sent them to the galleys for carrying forbidden titles. The author was never charged. He died in his bed on 21 January 1789 and was buried in a Catholic church, having never once admitted in public what he had written.
The admission came from Naigeon, and the attributions were first fixed in a bibliography of anonymous works in 1806. Scholars still argue over how much of the prose is Diderot's, and d'Holbach's hardest claim is still unsettled: if every choice has a cause, what is left of blame? For a long time he was filed as a populariser of other men's daring; the current reading, and a critical edition now being built at Oxford, treat his work as the most complete statement the radical Enlightenment ever produced.