If the women and men of the Enlightenment felt an affinity for the esprit systématique, a sense of the importance of complex networks of mutually defining, yet mobile, relationships, this affinity was reflected in and reinforced or perhaps even prompted by their participation in the widespread and engrossing practice of letter writing, beyond any encounters with trees of knowledge or Linnaean classificatory schemes. For us to understand the power of systematic reason and its critique, it is important to look beyond the disciplinary boundaries of philosophy and science. Sorting, naming, and classifying are not the exclusive province of the taxonomists. As I hope to show in this and subsequent chapters, the gulf between the love letters of Julie de Lespinasse and the abbé de Condillac's Traité de systèmes is not as wide as one might think.

Both social history and literary criticism have had a great deal to say in recent years about correspondence, real or fictional, as one of the eighteenth century's primary expressive forms. Like the salon conversation with which it was closely allied, letter writing was both a personal and a public performance, a means of affirming the importance of social relations, and a vehicle for furthering the exchange of ideas, the imparting of information, and the construction of a self, beyond the contacts immediately available to one. This is ground on which over-sharp distinctions of “public” and “private” founder. The most private, inward-looking letter is, as Terry Eagleton has remarked, “ineradicably social.”