Interactive Dualism

The primary problem of Dualism—many would say the defeater of Dualism—is how nonphysical substances could possibly interact with physical substances, especially given the common assumption that the physical world is a closed system. Also called the "pairing problem," after Kim's formulation of it (Kim, 2001, 2005), how could an immaterial thing, the mind, interact with a material thing, the body (or brain)? Notwithstanding our folk perception that the physical world affects my mind through my senses and my mind affects the physical world through my actions, most scientists and philosophers deny this is what is in fact happening. There would be no commonalities between physical and nonphysical substances, no means of exchange—the problem of mental causation on steroids. Moreover, if nonphysical substances could somehow affect and alter physical substances, wouldn't that require a transference of energy, and wouldn't such an addition violate the sacrosanct physical law of the conservation of energy? (Section: Robinson, 2023; Interactionism, 2023).

The Interaction Problem

The objection is as old as the position. Princess Elisabeth of Bohemia put it to Descartes in 1643, asking how an unextended thinking substance could determine the animal spirits to produce voluntary motion, and she pressed the point through several rounds against replies she found evasive (Shapiro, 2007). Kim has since argued that this is not merely a puzzle for dualists to solve but a demonstration that it cannot be solved: causation between distinct objects requires a shared space-like coordinate system that locates them relative to each other, souls are not in space, and therefore no soul causes anything (Kim, 2005). What makes this the pairing problem rather than a general worry is its precision. Even granting that a soul could move a body, nothing explains why soul A moves body A rather than body B, since whatever relation is supposed to pair them cannot be spatial.

It is worth separating two objections that are often run together. Elisabeth's is a contact objection, resting on the pre-Newtonian assumption that causation requires touching. The conservation objection is different and later, originating with Leibniz in the 1690s and becoming the dominant form only in the late nineteenth century (Pitts, 2020). Advocates of Interactive Dualism can fairly reply that the first rests on archaic billiard-ball physics, where causation requires hard substances to be in physical contact. The second requires a different answer, and the serious literature is about that.

Why Be an Interactionist at All

Advocates of Interactive Dualism (not that there are many among scientists and philosophers) say they have resources. They reject the weak dualism of Epiphenomenalism where the physical affects the mental but the mental does not affect the physical. But rejecting epiphenomenalism is not merely a preference; Popper argued that it carries an evolutionary cost that its holders have not paid. If the physical world is causally closed, conscious experience can have no independent effect on behavior. A property with no behavioral effect cannot be selected for. Therefore, consciousness could not have evolved (Popper, 1978). Popper pressed this against Huxley's epiphenomenalism and then against the identity theory, on the ground that for identity theorists the order of ideas does no evolutionary work. The argument concerns phenomenal states directly, since it is the felt character of pleasure and pain that Popper takes to be doing the selecting. This is the positive case: not that interaction is unproblematic, but that its denial has a bill of its own.

The argument is not decisive, and identity theorists reply that the mental order is selected precisely because it is the physical order under another description. But it explains why interactionism keeps returning despite the interaction problem, and it is now routinely invoked in the mental-causation literature by philosophers who are not dualists at all.

Quantum Openings?

Quantum mechanics, on the other hand, allows for various, albeit speculative ways, for the mental to affect the physical, even beyond the classic but controversial view that an "observer" is needed to "collapse" the wave function. Moreover, because quantum mechanics introduces fundamental uncertainty into the universe, and if by this, indeterminism holds, nonphysical substances might enjoy "wiggle room" to effect causation.

The most fully developed version is not a gesture but a specific neurophysiological proposal. Eccles, with the physicist Friedrich Beck, argued that mental intention raises the quantum probability of vesicular exocytosis at the presynaptic grid, so that a nonphysical mind biases which synapses fire without transferring any energy, on the ground that altering a probability distribution is not doing work (Beck and Eccles, 1992). Eccles paired each unitary mental experience, a "psychon," with a "dendron," a bundle of apical dendrites of cortical pyramidal cells. This is developed on the Landscape as Beck-Eccles's Quantum Processes in the Synapse. The objection is that it either introduces causation without a law to govern it, or leaves wholly unexplained what a probability-altering nonphysical agent could be.

Energy, Conservation, and Locality

Advocates can also appeal to different kinds of ethereal forces or energy transference systems. Perhaps mental powers can influence the distribution but not the quantity of energy in the brain—this is precisely the Beck–Eccles move, and it does seem a bit of a cheat. Perhaps each individual brain is not a causally closed system so that the conservation of energy need not apply.

Here the strongest defense is technical rather than speculative, and it belongs to Pitts, whose position appears on the Landscape as Pitts's Interactionist Dualism. His argument is that the conservation objection rests on features of conservation laws that its users have not examined (Pitts, 2020). Conservation is local, holding not for the universe as a whole but for each region separately: the energy in any volume changes only through flow across its boundaries. Constant total energy for the universe requires that the global sum of local conservation laws converge, which it probably does not. And by Noether's first theorem and its converse, conservation holds if and only if the laws are time-translation symmetric. Should symmetry fail at particular times and places because of nonphysical influence, conservation would fail there and then while continuing to hold in refrigerators and stars. Pitts's conclusion is that expecting conservation to hold inside the brain, without having looked, simply assumes that Cartesianism is false; Leibniz's objection therefore begs the question. Whether conservation does hold in brains is then an empirical question for neuroscience rather than a matter settled a priori. Cucu and Pitts (2019) separately sort which dualist responses to the conservation objection actually work and which are confused, and Montero (2006) and Collins (2008) reach related conclusions from different directions.

Because the whole idea of a closed physical system is based on the assumption that there are no nonphysical forces involved, wouldn't this assumption undermine the argument against interaction by making it circular? Pitts's answer is that it does, though only against the a priori version of the objection. Then there is "overdetermination," where mental and physical factors can each, independently, affect actions—an approach that, while possibly solving one problem, creates other problems (Robinson, 2023; Interactionism, 2023). Perhaps one could also just blow away the interaction problem by asserting that in systems that have minds, the law of conservation of energy is false. Perhaps because downward causation goes to the lower physical levels and emergence is enabled, the causal completeness of physics is wrong (Ellis, 2019).

A Caution About Modern Physics as a Loophole

Perhaps causal closure for the entire universe is also a nineteenth century invention, based on classical thermodynamics and statistical mechanics, which are now superseded by quantum mechanics, general relativity, dark matter, dark energy, and who knows what else? (I/RLK can make up another. Since string theory offers, depending on flavor, 10, 11 or 26 "compactified" extra dimensions, why couldn't nonphysical substances work via these extra dimensions? I can conceive of a precedent for this. To account for the "hierarchy problem" in physics, where gravity is vastly weaker than the other fundamental forces, some postulate that gravity "leaks" or "bleeds" into these extra dimensions.) It gets crazy.

It also gets, on at least one count, backwards. Some authors have proposed that because gravitational energy is not localizable in General Relativity, there are no physically meaningful local conservation laws for minds to violate, so relativity supplies the loophole. Pitts (2022) argues the reverse: General Relativity formally has an infinity of rigid symmetries of the action and hence, by Noether's theorem, an infinity of conserved energies-momenta, with the result that in the simplest case Cartesian mental influence would have to be spatio-temporally constant and therefore zero. Far from providing an escape, relativity makes mental causation harder. That the most technically serious defender of interactionism reaches this conclusion is worth registering: the appeal to unsettled physics is not a resource the position can draw on indiscriminately.

A Worked Empirical Attempt

Popper and Eccles's The Self and Its Brain (1977) remains the most sustained effort to make interactionism answer to neuroscience, and it appears on the Landscape as Popper and Eccles's Interactionist Dualism. Popper contributed the three-world ontology, in which World 2, the domain of conscious states, is the sole channel through which World 3, the objective contents of thought, can act on World 1. Eccles contributed the claim that a self-conscious mind actively scans and reads out from "liaison areas" of the dominant cerebral hemisphere, and, more strikingly, that the unity of the phenomenal field is conferred by the mind rather than achieved by the brain, inverting the usual binding problem. The book is the standard modern reference for interactionism and, correspondingly, the standard target. Its weaknesses are instructive for the position generally: the interaction mechanism was progressively refined but never made independently testable, and the split-brain interpretation underpinning Eccles's cortical localization has not held.

Theological Responses

Finally, there is always a theological solution. God can help. God could have created souls with powers, especially since "real" (i.e., libertarian) free will is an essential part of "God's plan," such that neither conservation of energy nor determinism holds, at least with respect to minds. (It is challenging how even God could make this coherent.)

Christian philosopher William Lane Craig describes himself as a "dualist-interactionalist" in that "the brain is itself part of... the physical reality with which the soul immediately interacts” (Craig, 2015). He argues that even though souls do not have spatial locations, "the question becomes why we should think that only spatial relations can pair a cause with its effect. Prima facie this seems overly restrictive" (Craig, 2023). Craig's reply targets Kim's premise rather than his inference, and he is not alone: Bailey, Rasmussen and Van Horn (2011) argue that both of Kim's foundational claims—that causation requires spatial relations, and that souls cannot be in space—can be rejected.

I mustn't forget "Occasionalism," the idea that created substances, physical and nonphysical, cannot be efficient causes of events in themselves and that all events are caused directly by God. Its fullest development is Malebranche's, and it appears on the Landscape as Malebranche's Occasionalism. This means that while mind and body appear to interact, God is in fact changing each separately and ceaselessly. Two features of Malebranche's version are usually missed when it is dismissed. First, it is not a patch applied to the mind-body case: Malebranche denies causal power to bodies acting on bodies just as firmly, on the general ground that a true cause is one between which and its effect the mind perceives a necessary connection, and that we perceive such a connection only between the will of an omnipotent being and what it wills (Malebranche, 1674–75/1997). The mind-body problem is therefore not a special embarrassment but an instance of a difficulty he takes to be everywhere. Second, occasionalism does not solve the interaction problem; it denies that any interaction occurs, which is a different and more radical move. The argument from necessary connection later passed to Hume, who kept the analysis of causation and discarded the God.

While Occasionalism is dismissed (often ridiculed), there is a kind of logic here. If God acts as intermediary, as it were, between nonphysical and physical substances, then because God would have created both in the first place, this would make the apparent causal connection between nonphysical and physical substances not especially troublesome for God to bring about. This way of thinking—all these possible mechanisms for Interactive Dualism—reflects the depth of Dualism's problem.

References

[Note: existing citations in the entry—Robinson, 2023; Interactionism, 2023; Ellis, 2019; Craig, 2015, 2023—should be merged with the entries below from the live reference list.]

Bailey, A. M., Rasmussen, J., and Van Horn, L. (2011). "No Pairing Problem." Philosophical Studies 154(3): 349–360.

Beck, F., and Eccles, J. C. (1992). "Quantum Aspects of Brain Activity and the Role of Consciousness." Proceedings of the National Academy of Sciences USA 89(23): 11357–11361.

Collins, R. (2008). "Modern Physics and the Energy-Conservation Objection to Mind-Body Dualism." American Philosophical Quarterly 45(1): 31–42.

Cucu, A. C., and Pitts, J. B. (2019). "How Dualists Should (Not) Respond to the Objection from Energy Conservation." Mind and Matter 17(1): 95–121.

Kim, J. (2001). "Lonely Souls: Causality and Substance Dualism." In K. Corcoran (ed.), Soul, Body, and Survival: Essays on the Metaphysics of Human Persons. Ithaca: Cornell University Press, 30–43.

Kim, J. (2005). Physicalism, or Something Near Enough. Princeton: Princeton University Press.

Malebranche, N. (1674–75/1997). The Search after Truth. Trans. and ed. T. M. Lennon and P. J. Olscamp. Cambridge: Cambridge University Press.

Montero, B. G. (2006). "What Does the Conservation of Energy Have to Do with Physicalism?" Dialectica 60(4): 383–396.

Pitts, J. B. (2020). "Conservation Laws and the Philosophy of Mind: Opening the Black Box, Finding a Mirror." Philosophia 48(2): 673–707.

Pitts, J. B. (2022). "General Relativity, Mental Causation, and Energy Conservation." Erkenntnis 87(4): 1931–1973.

Popper, K. R. (1978). "Natural Selection and the Emergence of Mind." Dialectica 32(3–4): 339–355.

Popper, K. R., and Eccles, J. C. (1977). The Self and Its Brain: An Argument for Interactionism. Berlin: Springer International.

Shapiro, L. (ed. and trans.) (2007). The Correspondence between Princess Elisabeth of Bohemia and René Descartes. Chicago: University of Chicago Press.