1 CHAPTER 11 There is no Exclusion Problem STEINVÖR THÖLL ΆRNADΌTTIR & TIM CRANE 0. Introduction Many philosophers want to say both that everything is determined by the physical and subject to physical laws and principles, and that certain mental entities cannot be identified with any physical entities. The problem of mental causation is to make these two assumptions compatible with the causal efficacy of the mental. The concern is that this physicalist picture of the world leaves no space for the causal efficacy of anything non-physical. The physical, as it is sometimes said, excludes anything non- physical from doing causal work. The general shape of the problem is not new. Leibniz famously argued that Descartes’s conception of the relationship between mind and body had no place for mental causation. On Descartes’s view, according to Leibniz, the mind can only affect the body by changing the ‘direction of motion’ of the body’s ‘animal spirits’. Descartes had held that in this way the total ‘quantity of motion’ was conserved in psychophysical interaction. But Leibniz claimed that what should be conserved in these interactions is not quantity of motion but (as we would now put it) quantity of momentum (mass times velocity). So the mind cannot alter the direction of motion of the animal spirits without altering the quantity of momentum in the physical world. The physical law that Leibniz took himself to have discovered excludes the mental from making a causal difference. 1 Leibniz’s objection to Descartes was based on his view of the nature of the physical world. Contemporary philosophers also see the problem of mental causation as arising from assumptions about the physical world (see Papineau 1990). Partly because of the need to accommodate mental causation given these assumptions, many philosophers have sought to find a more intimate connection between the mental and the physical, holding that mental entities are determined by or constituted by physical 1 For Leibniz’s views, see Leibniz 1695, 1696. For contemporary discussion, including of the question of whether Leibniz’s had correctly interpreted Descartes, see Garber 1983, and Woolhouse 1985.