Leibniz's correspondence with the Jesuit theologian and philosopher Bartholomew Des Bosses is one of the richest sources of Leibniz's mature philosophical thought. Occupying the last ten years of Leibniz's life (1706–16), the period of composition of the Theodicy (1710), the Monadology (1714), and the Principles of Nature and Grace (1714), and comprising close to 140 letters, this correspondence shows Leibniz confronting some of the most difficult questions concerning his metaphysics: the nature of composite substance and the reality of relations. Moreover, because the correspondence was never meant to be published, it offers us a unique and salutary perspective on Leibniz's philosophy, for it is here that Leibniz is at his most relaxed, working out ideas in his system.

For many scholars, however, the Leibniz–Des Bosses correspondence is inextricably linked with that “enigma” (Blondel 1930) of modern philosophy, the vinculum substantiale, or substantial bond of monads – that which is apparently invoked by Leibniz to explain transubstantiation within his monadological metaphysics. It should not be surprising, then, that Bertrand Russell's well-known, damning remark about the vinculum substantiale – that it was “rather the concession of a diplomatist than the creed of a philosopher” (1937: 152) – has cast a pall over much of the scholarship relating to the Leibniz–Des Bosses correspondence. Russell's view, pushed to its extreme, seemed to be that the entire correspondence constitutes simply an attempt on Leibniz's behalf to placate his benighted, Catholic friend, and, as such, that we hardly need to concern ourselves with any of Leibniz's arguments with Des Bosses.