  While separate entries detail Leibniz's account of causation and his account of the mind, it will still be useful to provide a short exegesis of Leibniz's celebrated solution to the mind-body problem which Leibniz had inherited from Descartes and his followers. The problem, briefly, is this: if mind is essentially thought (and nothing else), and body is essentially extension, then how can mind and body interact or form a unity as we know from experience they must? Or how do thinking substance and extended substance unite in the substance of a human being? Leibniz answers this question by, first, denying the possibility of the causal interaction of finite substances. In this way, Leibniz undermines Cartesian dualism because it takes as a premise the idea that mind-body interaction is to be explained by the influence of the one on the other via the pineal gland. (See the Sixth Meditation: AT VII 86–87/CSM II 59–60) But Leibniz also saw pre-established harmony as an account of the mind-body relation that avoided the difficulties inherent in Occasionalist theories of the mind and the interaction of substances. In one of Leibniz's best-known metaphors, he asks his readers to imagine the mind and body as two pendula hanging from a beam. Whence comes their agreement? One could imagine that the motion of the one is communicated through the wooden beam to the other, thus causing them eventually to swing harmoniously (the theory of influx). Or one could imagine that God intervenes and moves the pendula, guaranteeing their synchronicity (the theory of occasionalism). Or, Leibniz says, one could imagine that God, the supreme artificer, created the world (and the pendula) so perfectly that, by their own natures, they would swing in perfect harmony. Naturally, it is this last thesis that Leibniz endorses and asks his readers to endorse as well. (See, for example, the Postscript of a Letter to Basnage de Beauval (G IV 498–500/AG 147–49).)