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.