This chapter explores the possibilities for and limitations upon social mobility as Defoe conceived them. His own biography exemplified the roller-coaster trajectory of the first-phase professional writer. In his non-fictional prose, he showed that commoners without pedigree, land, or title could aspire to the highest social echelons. His fiction, though, was energized by the opposite trajectory: downward mobility. His protagonists live on the paper-thin line between respectability and criminality, terrified of falling off the edge. The conflict between an absolute system of morality as prescribed in the Ten Commandments and the relative morality by which people in the world must live is one experienced repeatedly by Moll Flanders and other Defovian characters. Understanding the pragmatic ethical codes to which such characters must resort does not, though, entirely explain their fictional fates. Narrative energy is produced by the gap between the explicable and the mysterious in his characters’ motivations and actions.