The enduring metaphysical question about minds and mental phenomena concerns their nature. At least since Descartes this question—the mind-body problem—has been understood in terms of the viability or necessity of mind-body dualism, the thesis that minds and bodies are essentially distinct kinds of substance. Assuming that the nonmental (‘body’) portions of the world are constituted of physical stuff, the remaining question is: Are minds or mental phenomena essentially distinct non-physical substances, or phenomena that essentially involve such distinct kinds of substances? By the middle of the Twentieth Century there was broad philosophical and scientific consensus that the answer to this classical question about minds is negative: Minds and mental phenomena are not essentially distinct substances, nor are they phenomena that essentially involve distinct kinds of substances. There are at least two broad trends and one specific argument that lead to this conclusion. One trend is the decreasing influence of specifically theological arguments and commitments in philosophical argumentation, so that religious belief in immortal souls was no longer given much weight in the ontology of mind. The second trend, perhaps related to the first, is the increased demand that metaphysical theories bear explanatory fruits, so that the postulation of an immaterial and essentially mental substance appears to be a abdication from explanatory duties rather than a useful proposal. The argument, known to Descartes from the very beginning, is that there has never been an adequate account of 1 how two essentially distinct and incompatible substances could causally interact.1 Descartes’ solution was inadequate and brute, and his followers struggled with the problem—leading to Leibniz’s parallelism and Malebranche’s occasionalism, among other views. The problem of mental causation, then, is the central difficulty that underines substance dualism.2 The negative answer on the question of substance dualism, however, only increases the pressure for some monistic account of the nature of minds...