Giving an account of mental causation — in particular, explaining how it is possible for the mental to causally affect the physical — has been one of the central problems in the philosophy of mind over the past decade or so. The problem of course is not new: Descartes famously was confronted by many of his contemporaries — for example, Gassendi and Princess Elizabeth of Bohemia — with the same question. But this does not mean that Descartes' problem is our problem. For his problem, as his contemporaries saw it, arose from his substantival dualism, a dualism of material and mental substance. But, at least for most of us, that is not the source of our worries about mental causation. Few of us now believe in the existence of substantival minds or some kind of mind-stuff that is ontologically independent of material bodies.