This chapter discusses a particularly important objection to substance dualism, that there is something problematic about the idea that immaterial souls can causally interact with physical bodies. This objection is best put in terms of the pairing problem for substance dualism, which claims that substance dualism is objectionable because it would result in souls and bodies being causally paired in an objectionably brute manner. I argue that those who think that we are composite physical objects face an analogous problem: the mereological pairing problem. According to the mereological pairing problem, the thesis that we are composite physical objects is objectionable because composite physical persons and their parts would be paired in an objectionably brute manner. The upshot of the chapter is that one of the most prominent objections to substance dualism is indecisive at best, since it can be parodied.

The chapter uses the principle according to which difference-making or counterfactual dependence is sufficient for causation to show that there are physical effects of mental causes. If non-reductive physicalism is true, applying the principle is straightforward. The principle also yields higher-level causes that are not mental but might be considered problematic. These causes are best diagnosed as causes that have little explanatory relevance. If dualism is true, applying the principle about causation in order to show the existence of mental causation is less straightforward, but still possible. In order to avail themselves of the principle, dualists need to assume that the laws that connect the mental and physical realms have a special status. Rival approaches according to which mental causation or human agency require the transference of a physical quantity or of a power are in conflict with empirical results. The account of mental causation by counterfactual dependence, by contrast, squares with these results.

The introduction describes the main problems of mental causation, their interrelations, and their history. The first problem is the interaction problem, the problem of how the mind and the physical world can interact at all. The second problem is the exclusion problem, the problem of how the mind can have physical effects given that these physical effects already have physical causes. How severe the problems are depends on the nature of the mind. The more intimate the relation between the mental and the physical, the more pressing the problems become. How severe the problems are also depends on the nature of causation. If causation requires the transference of a physical quantity, the problems are much harder to solve than if it suffices for causation if the cause makes a difference to the effect. The introduction outlines the history of the problems from Descartes to the twentieth century.