The Leibniz Review

Leibniz denies that the actual world possesses the per se unity of a substance. Instead, he seems to hold, the world is limited to the mind-dependent unity of an aggregate. Against this answer, criticized by Kant in his Inaugural Dissertation, I argue that for Leibniz the unity of the actual world is not grounded simply in God’s perception of relations among created substances but in the common dependence of those substances on a unitary cause. First, the actual world is one because every created substance is continuously dependent on God for its perfection. Without being the soul of the world, God is an emanative cause through which the created world is unified. Second, every substance is a unique “concentration” of an ideal world that is God’s model for creation. Consequently, while extensionally many, created substances are versions of the same one world chosen by God for creation.

In a 1687 letter to Arnauld, Leibniz draws on an argument against mind-body causation that is reminiscent of one from Spinoza’s Ethics. According to this argument, mind-body causation is impossible because of the lack of proportion between thoughts and motions. This paper aims to shed light on Leibniz’s use of Spinoza’s argument by reconstructing both its internal structure and its development in Leibniz’s later works. In particular, the reconstruction focuses on the new version of this argument that Leibniz adopts against Stahl’s vitalism as well as on the change that this new version reveals in Leibniz’s attitude towards occasionalism. The possible influence of Cordemoy is also taken into consideration. The epistemological and metaphysical issues surrounding this argument are an essential part of the history of Leibniz’s psycho-physical parallelism.