 A central philosophical issue of the seventeenth century concerned the apparent causal relations which hold between the mind and the body. In most seventeenth-century settings, this issue was discussed within the context of substance dualism, the view that mind and body are different kinds of substance. As noted above, Leibniz remained fundamentally opposed to dualism. But although Leibniz held that there is only one type of substance in the world, and thus that mind and body are ultimately composed of the same kind of substance (a version of monism), he also held that mind and body are metaphysically distinct. There are a variety of interpretations of what this metaphysical distinctness consists in for Leibniz, but on any plausible interpretation it is safe to assume (as Leibniz seems to have done) that for any person P, P’s mind is a distinct substance (a soul) from P’s body. With this assumption in hand, we may formulate the central issue in the form of a question: how is it that certain mental states and events are coordinated with certain bodily states and events, and vice-versa? There were various attempts to answer this question in Leibniz’s time period. For Descartes, the answer was mind-body interactionism: the mind can causally influence the body and (most commentators have held) vice-versa. For Malebranche, the answer was that neither created minds nor bodies can enter into causal relations because God is the only causally efficient being in the universe. God causes certain bodily states and events on the occasion of certain mental states and events, and vice-versa. Leibniz found Descartes’ answer unintelligible (cf. Theodicy, sec. 60; G VI, 135f.) and Malebranche’s excessive because miraculous (cf. Letter to Arnauld, 14 July 1686; G II, 57/LA 65).