Diderot and Rousseau were friends and then enemies, and they were also both major writers of the Enlightenment. They argued that human nature should be understood and valued, and they argued against anything that constrained it, as they considered that all suffering was destructive. Fiction was part of their argumentative arsenal, and perhaps even the tool they felt was most effective, as it works through the imagination on the emotions. 'Natural' reactions of dismay or distress at injustice or cruelty could 'enlighten' the reader at an emotional and therefore natural level, and create new ways of seeing that rejected harsh convention and promoted natural morality. This chapter tracks these aspects through their fictional and non-fictional works, showing how central they are to all their writing. We also look at the friendship of these two writers, and at the publication history of their fictional work.