Descartes's lack of clarity about the causal connections between brain states and mental states has led many commentators to conclude that he has no coherent account of body-mind relations in sensation, or that he was simply confused about the issue. In this paper I develop what I take to be a coherent account that was available to Descartes, and argue that there are both textual and systematic reasons to think that it was his considered view. The account has brain states serving as occasions for the mind to produce in itself the sensations that it takes these brain states to signify. The relation between body and mind on this model is thus neither a standard efficient-causal relation, nor an occasionalist one, but rather a semantic-caus...