 Scholars have also suggested, though never proven definitively, that Diderot contributed during these years to Baron d’Holbach’s Système de la Nature, first published in 1770, a book that stands alongside Helvétius’ De l’Esprit as one of the great masterpieces of French Enlightenment materialist philosophy. Diderot was certainly at the center of the D’Holbach’s coterie, and if the dry and programmatic systematicity of D’Holbach’s book lacks the lively play of Diderot’s best philosophical writing, it is certain that he and the Baron were kindred spirits.