Descartes’ division between mind and body depended on the existence of two separate substances: mind and material body, in addition to God. For Spinoza, there was but one substance, which was also God. That is, the human mind and the human body are the same exact thing, but are understood in different ways. We do not think of one thing as interacting causally with itself. So Cartesianism could not even get started as a problem in Spinoza’s system.