In my last post, I discussed an alleged inconsistency between interactionist substance dualism and our most successful theories of fundamental physics, referred to collectively as “Core Theory” (a term coined by Sean Carroll, I believe), and I concluded that there is no necessary inconsistency between the two. According to my argument, if mind-body interaction occurs, then the physical effects of mind-body interaction will already be among the phenomena that Core Theory describes. The fundamental laws of physics are, in other words, determined in part by the interaction of mind and body, implying that interaction between mental and physical substance occurs everywhere that our fundamental laws of physics apply.
This implication may seem hard to believe; however, in order to avoid violating Core Theory themselves, physicalists too must accept something very much like it. Physicalists believe that there is only physical stuff; mind is viewed as a product of or identical to a certain kind or arrangement of physical substance (e.g. a human brain) — it is a complex but fundamentally physical phenomenon. With respect to the relation between basic physical phenomena and mental phenomena, many physicalists endorse some kind of emergentism, i.e. the view that mental phenomena emerge from suitably arranged basic physical entities, but are not reducible to or governed by the same basic laws as those entities. Emergence purports to explain why it seems so hard to explain how the mind could be constituted by the kinds of basic entities — quarks, photons, electrons, etc. — that populate our fundamental theories of physics: suitably arranged, certain complex physical objects come under the jurisdiction of a different set of fundamental physical laws. Because these laws are fundamental, they cannot be inferred from any amount of knowledge about lower-level physical entities and the rules they play by. Emergentism also offers an escape route from the threat of epiphenomenalism, the view that mental states make no difference to anything that happens in the brain; emergent mental states have causal powers over and above the causal powers of the physical entities from which they emerge.
Unfortunately, the emergentist variety of physicalism seems to face the same problem as interactionist dualism. The worry is that if distinct causal powers emerge in the brain, the behaviour of matter in the brain will not conform to the predictions of Core Theory. Perhaps, on closer study, it will turn out that there are local violations of Core Theory in the brain. If so, emergentists are off the hook, but so are interactionists, and physicalism loses a major advantage over dualism. If, as I think is more likely, no violations occur, emergentists are in a tight spot. Assuming epiphenomenalism is unacceptable, then any independent causal role for consciousness must already be accounted for in the fundamental laws of Core Theory. This implies that the fundamental laws of Core Theory are determined in part by the causal contribution of mental states emerging from physical substance, and the apparent uniformity of these laws across space and time implies that the effects of emergent mental states pervade time and space. The bottom line is that in order to be consistent with our best physics, a plausible kind of physicalism must — for almost the exact same reasons as dualists — take mental phenomena to be partly constitutive of fundamental natural laws as described by science.
See section 8 of “Consciousness and its Place in Nature” by David Chalmers for an alternative formulation of the problem I discuss in the third paragraph.