This essay argues that the structural differences among the three parts of Aphra Behn's Love-Letters between a Nobleman and his Sister (1684-87) are the result of Behn's increasingly complex challenge to changing notions of the self, arising out of the philosophical (or 'scientific') revolution. Behn shifted from epistolarity to nearly full narration in search of a narrative structure that would convey both her skepticism about the unified, stable self, posited by natural philosophers such as Robert Boyle, and her view of the self as unstable and unknowable. Behn recognized the dangers of an epistemology based on the illusion of this self propounded by Fellows of the RoyalSociety, perils epitomized by the political and social crises of the 1680s leading to the Duke of Monmouth's rebellion.