 To answer this question one would need to delve deeply into Leibniz’s long and fascinating correspondence with Des Bosses (as well as other texts). This is obviously not the place to provide such an investigation, but we can briefly summarize a standard interpretation of one of the central moments of the correspondence (see Look and Rutherford’s introduction [2007] to the Leibniz-Des Bosses Correspondence, xlix–lxxviii). As a Jesuit priest, Des Bosses was interested in determining whether Leibniz’s philosophy could provide an adequate ontological framework for the doctrine of transubstantiation. The central point of contention in their discussion of this issue concerned the reality of corporeal substance. Did Leibnizian corporeal substances meet the criteria for being a substance in the strict sense of the term? On a monadological view a corporeal substance (or animal) is a dominant monad united with an organic body, which is itself an aggregate of subordinate monads. The “union” of a dominant monad and its organic body consists in certain harmonious perceptual relations that obtain between the dominant monad and the subordinate monads of the organic body. This rather weak notion of union is not sufficient to make the dominant monad and its organic body truly one being. This implies that a corporeal substance is not a substance strictly speaking because it does not possess per se unity. Des Bosses thought that this reductive view of corporeal substance did not provide a robust enough conception of the reality of bodies to properly ground the doctrine of transubstantiation. In response to Des Bosses’ concerns Leibniz developed the notion of a substantial bond (vinculum substantiale). This substantial bond (superadded by God) could serve as a kind of metaphysical glue uniting the dominant monad and the subordinate monads of an organic body. Leibniz told Des Bosses that the only way for a corporeal substance to be a per se unity would be through one of these substantial bonds. Arguably, Leibniz did not endorse the theory himself; he told Des Bosses that he preferred the more ontologically parsimonious version of the theory of monads (letter of 16 June 1712; LR 255).