In this work, Henry Vyverberg traces the evolution and consequences of a crucial idea in French Enlightenment thought--the idea of human nature. Human nature was commonly seen as a broadly universal, unchanging entity, though perhaps modifiable by geographical, social, and historical factors. Enlightenment empiricism suggested a degree of cultural diversity that has often been underestimated in studies of the age. Evidence here is drawn from Diderot's celebrated Encyclopedia and from a vast range of writing by such Enlightenment notables as Voltaire, Rousseau, and d'Holbach. Vyverbergexplains not only the age's undoubted fascination with uniformity in human nature, but also its acknowledgment of significant limitations on that uniformity. He shows that although the Enlightenment's historical sense was often blinkered by its notions of a uniform human nature, there were alsocracks in this concept that developed during the Enlightenment itself.