Examining the educational writings of three of the eighteenth-century's most innovative thinkers, the Abbé de Saint-Pierre, Morelly and Helvétius, this article challenges the currently accepted view that it was a belief in human pliability which gave rise to the contemporary groundbreaking faith in the power of education to improve society. The article delineates an intellectual process that culminated in the stance that man's innate behavioural tendencies are unalterable. It argues that, at least prior to Rousseau, the eighteenth-century faith in the power of education to improve society rested on a conviction that it is possible to beneficially direct man's fixed behavioural tendencies.