  This conclusion in the Sixth Meditation asserts the well-known substance dualism of Descartes. That dualism leads to problems. As Princess Elisabeth, among others, asked: if mind is unextended and matter is extended, how do they interact? This problem vexed not only Descartes, who admitted to Elisabeth that he  didn't have a good answer (3:694), but it  also vexed Descartes' followers and other metaphysicians. It seems that, somehow, states of the mind and the body  must be brought into relation, because when we decide to pick up a pencil our arm actually moves, and when light hits our eyes we experience the visible world. But how do mind and body interact? Some of  Descartes' followers adopted an occasionalist position, according to which God mediates the causal relations between mind and body; mind does not affect body, and body does not affect mind, but God gives the mind appropriate sensations at the right moment, and he makes the body move by putting it into the correct brain states at a moment that corresponds to the volition to pick up the pencil. Other philosophers adopted yet other solutions, including the monism of Spinoza and the pre-established harmony of Leibniz.