There is mental causation whenever a mental state, event, process, or activity has a causal effect. The pursuit of our lives seems replete with mental causation. It may thus seem as obvious that it occurs as we pursue our lives. But how mental causation is possible is not obvious. And therein lies a philosophical tale. Any attempt to explain how it occurs must engage the mind-body problem.

René Descartes (1596–1650) maintained that there is body-to-mind causation when we perceive our surroundings, and mind-to-body causation when we act. But one of the most serious charges leveled again his substance dualism, according to which the mind is an immaterial substance that is not extended in space, is that it leaves unexplained how mental states and events (etc.) have causal effects on our bodies. Descartes held that the locus of mind-body causal interaction is in the brain (specifically, in the...