Leibniz’s Appropriation of Spinoza’s Argument against Mind-Body Causation

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.