The Unity of Composite Substance: The Scholastic Background to the Vinculum Substantiale in Leibniz’s Correspondence with Des Bosses

This paper explores the scholastic context of the discussion about the unity of the composite or corporeal substance and the nature of the vinculum substantiale or substantial bond in Leibniz’s correspondence with Des Bosses. Three prominent scholastic views are examined: Duns Scotus’s antireductionist account of the composite substance as an entity irreducible to its essential parts (i.e., matter and substantial form); Ockham’s parts-whole identity thesis, which entails a reductionist view of the composite substance; and Suárez’s explanation of the unity of composite substance through the presence of a substantial mode of union. It is then shown that Leibniz initially combines a reductionist account of the composite substance, with the vinculum playing the role of bond among the component monads. In his last letters, he moves away from this to an antireductionist account of the composite substance, with which he now identifies the vinculum.