 One of the pressing questions in seventeenth-century philosophy, and perhaps the most celebrated legacy of Descartes’s dualism, is the problem of how two radically different substances such as mind and body enter into a union in a human being and cause effects in each other. How can the extended body causally engage the unextended mind, which is incapable of contact or motion, and “move” it, that is, cause mental effects such as pains, sensations and perceptions? And how can an immaterial thing like a mind or soul, which does not have motion, put a body (the human body) into motion? Spinoza, in effect, denies that the human being is a union of two substances. The human mind and the human body are two different expressions—under thought and under extension—of one and the same thing: the person. And because there is no causal interaction between the mind and the body, the so-called mind-body problem does not, technically speaking, arise.