Violence in Sade's published works exercises a powerful fascination on his readers and encourages them to believe that the characters in his novels voice the author's personal opinions. We can easily overlook the fact that Sade was not an ideologue but a writer of fiction. Analysing the ways in which Sade develops his characters, this article argues that Sade employed theories taken from the field of natural history in order to construct the speech and the actions of his characters. This article is a case study which examines Sade's literary exploitation in La Nouvelle Justine (1797) of certain theoretical positions deriving from J.-B. Robinet's treatise De la Nature (1761).