Popper and Eccles's Interactionist Dualism: Three Worlds and the Liaison Brain

Popper and Eccles defend interactionist dualism: phenomenal consciousness is a real, non-physical domain that both receives from and acts upon the brain, and the physical world is therefore not causally closed. Popper argues the philosophical case, positing that, from natural selection, conscious experience must have causal efficacy or it could not have evolved, and embedding mental states in a three-world ontology in which subjective experience mediates between physical states and the objective products of thought. Eccles supplies the neurophysiological mechanism, proposing that a self-conscious mind actively scans and reads out from specialized cortical modules and, crucially, that the unity of the phenomenal field is conferred by the mind rather than achieved by the brain. Neither author claims to explain why experience has qualitative character; their argument is that it exists, that it does causal work, and that materialism cannot accommodate either fact.

In other words, Popper and Eccles maintain that the now-minority view among philosophers and neuroscientists that phenomenal consciousness is ontologically irreducible to the brain, causally affected by neural activity, and capable of altering neural processes. In The Self and Its Brain, Popper offers his pluralist metaphysics of three interacting “worlds,” while Eccles attempts to identify a neurophysiological interface through which a nonphysical “self-conscious mind” selects, interprets, and unifies brain activity; in later work Eccles sharpens this into the “psychon–dendron” hypothesis and a speculative quantum mechanism of mental causation (Popper & Eccles, 1977; Eccles, 1990, 1994). Their theory is historically important not because its proposed mechanism has been embraced—it has not—but because it represents an ambitious attempt to combine irreducible subjective experience, causal mental agency, and neuroscience.

Two Authors, Two Arguments

The Self and Its Brain (Popper and Eccles, 1977) is structured in three parts: Popper's philosophical case, Eccles's neurophysiological case, and twelve recorded dialogues held at the Villa Serbelloni in 1974. Reviewers noted at the time that the result is not a single unified theory, since the two halves are not fully reconciled and the authors did not act as sharp critics of each other (Marriott, 1978). The convergence is real but partial: both hold that mental events are irreducible and causally efficacious, while Popper is a pluralist about worlds and cautious about mechanism, and Eccles is committed to a substantive self, a specific cortical locus of interaction, and eventually a personal survival thesis that Popper did not share. The collaboration had a genuine intellectual basis. Eccles had championed electrical synaptic transmission and, on his own account, was led by Popper's falsificationism to welcome the experiments that refuted him in favor of chemical transmission.

Popper's Three Worlds

Popper’s foundational reality comprises three “worlds.” He distinguishes World 1, physical objects and states; World 2, states of consciousness, subjective experience, and mental dispositions; and World 3, the objective contents of thought, comprising theories, problems, arguments, and artifacts, which are human products that acquire autonomy and unintended consequences once made (Popper, 1972). Phenomenal consciousness is World 2 and is treated as ontologically basic rather than derived. The interaction is asymmetric in an important way: World 3 can act on World 1 only through World 2. A theory changes the physical world only by being grasped by a mind. This gives subjective experience an indispensable structural role rather than an accompanying or incidental one, and it is the main move against epiphenomenalism. Popper further argues that the self is not simply given but is developed, anchored in language and in the World 3 objects a person comes to grasp, so that selfhood is an achievement rather than a primitive.

The Evolutionary Argument Against Epiphenomenalism

Popper's central argument is that epiphenomenalism and the identity theory are incompatible with natural selection (Popper, 1978). 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 applies this against Huxley's epiphenomenalism and then against identity theorists, on the ground that for them the order of ideas is evolutionarily redundant. The argument concerns phenomenal states directly, since it is the felt character of pleasure and pain that Popper takes to be doing the selective work. It is not original to him, having earlier forms in James and Romanes, but the Popper–Eccles formulation is the one that entered late-twentieth-century debate as the standard version. Popper links it to downward causation, the term introduced by Campbell (1974), which he adopts to describe higher-level structures constraining their constituents.

Eccles's Liaison Brain and the Unity of Experience

Eccles's proposal is that the self-conscious mind is not a passive product of neural activity but an active agent that searches, selects, attends to, and integrates the output of the cerebral cortex. Interaction occurs in "liaison areas," the open modules of the dominant, language-bearing hemisphere. He draws heavily on the split-brain findings, concluding that the self-conscious mind is linked to the dominant hemisphere alone. Sperry contested this directly: testing commissurotomy patients with lateralized visual input, he and his colleagues found that the disconnected minor hemisphere recognized the subject's own face, relatives, pets, and public figures, with emotional responses indicating a personal and social self-awareness roughly comparable to that of the major hemisphere in the same subject (Sperry, Zaidel and Zaidel, 1979; Sperry, 1984).

The claim most directly about phenomenal consciousness is Eccles's account of unity. Cortical activity is fragmented across modules; the phenomenal field is unified; and Eccles infers that the unity is imposed by the self-conscious mind rather than produced by neural integration. This inverts the standard binding problem: for Eccles, binding is not something the brain accomplishes but something the mind does to what the brain supplies.

Psychons, Dendrons, and the Energy Objection

The standing objection to interactionism is that a non-physical mind acting on neurons would violate conservation of energy. Eccles's later work is an attempt to answer it. He proposed that mental life is composed of unitary experiences he called psychons, each uniquely linked to a dendron, a bundle of apical dendrites of cortical pyramidal cells, with roughly forty million such pairs (Eccles, 1990). With the physicist Friedrich Beck he then argued that mental intention could raise the quantum probability of vesicular exocytosis at the presynaptic grid without any transfer of energy, since altering a probability distribution is not the same as doing work (Beck and Eccles, 1992). Eccles developed the proposal into a full account of mind-brain interaction in his final book (Eccles, 1994). Critics have replied that it either smuggles in causation without a law to govern it or leaves unexplained what a probability-altering non-physical agent could be (Rosenkranz, 1996). The proposal appears on the Landscape as Beck-Eccles's Quantum Processes in the Synapse.

Reception and Influence

The Popper-Eccles book became the standard modern reference for dualist interactionism and, correspondingly, the standard target for those seeking to defeat it. Churchland (1984) treats it as the position against which materialist alternatives are defined. Its most durable legacy runs through the causal efficacy of the mental rather than through dualism as such. The evolutionary argument against epiphenomenalism is now routine in the mental-causation literature and is invoked well outside dualist quarters. Popper's downward causation, coupled with strong emergence, is developed on the Landscape by Ellis's Strong Emergence and Top-Down Causation, and Sperry's Mentalist Paradigm advances a closely allied claim about the causal potency of emergent conscious states while explicitly rejecting dualism. Hasker's Unified Experience entry pursues the unity argument toward emergent rather than substance dualism, and Pitts's Interactionist Dualism engages the conservation objection directly. Popper's World 3, in which cognition is partly constituted by external, publicly available structures, structurally anticipates later externalist accounts of mind, though the historical line to A. Clark and Chalmers's Extended Mind is one of resemblance rather than documented influence. A conspicuous omission, noted by reviewers, is that the book barely engages the computational theory of mind that was ascendant when it appeared.

Popper also coined the term “promissory materialism” in The Self and Its Brain for a recurrent strategy in some materialist theories of mind: acknowledging that contemporary neuroscience cannot explain consciousness while treating future scientific progress as effectively guaranteeing that a wholly physical account will eventually succeed. In his section “The New Promissory Materialism,” Popper’s target is not empirical neuroscience or reduction as a research program—he explicitly favored attempted scientific reductions—but the conversion of anticipated future discoveries into support for present metaphysical certainty. He describes a projected future in which physiological vocabulary progressively displaces talk of experiences, perceptions, thoughts, beliefs, purposes, and aims, until the mind–body problem is deemed to have disappeared (Popper & Eccles, 1977). Eccles then made the expression a recurring polemical theme. Explicitly crediting Popper with the label, he called the claim “extravagant and unfulfillable” and later characterized promissory materialism as a dogmatic “superstition” resembling a messianic prophecy (Eccles, 1979, 1994). The phrase has since become a compact criticism of arguments that infer ontological materialism from the expected—not yet achieved—explanatory success of future neuroscience.

Explanatory Power and Limits

Popper and Eccles refuse to identify subjective experience with behavioral function, computation, or neural activity merely because those domains correlate, and they confront the causal efficacy and unity of consciousness directly. Their collaboration is methodologically unusual: a major philosopher of science and a Nobel neurophysiologist attempted to connect metaphysics of mind with cortical physiology. Eccles’s standing as an experimental neuroscientist was exceptional—his work on excitation and inhibition at synapses earned him a share of the 1963 Nobel Prize in Physiology or Medicine.

The central weakness is equally apparent. The theory posits the self-conscious mind whose powers of selection, integration, phenomenal realization, and causal intervention most require explanation. Dennett’s early criticism was that the account hands difficult integrative work to the self-conscious mind without explaining how it performs that work (Dennett, 1979). Eccles’s later quantum hypothesis proposes a microscopic locus for the interaction but does little to address, let alone solve, this explanatory problem. The theory therefore remains a major modern statement of interactionist dualism and a historically significant challenge to reductive materialism, but its claimed mind–brain mechanism remains unambiguously conjectural.

Assessment

The Popper–Eccles position is strongest as a negative argument and weakest as a positive one. The evolutionary case against epiphenomenalism identified a genuine cost of causal closure that its opponents still must answer, and Eccles's observation that phenomenal unity outruns demonstrated neural integration named a problem that remains open. What neither author supplies is an account of why any state, physical or mental, should have felt character at all; World 2 is posited rather than explained, and psychons are drawn-up labels for experiences rather than analyses of them. The interaction mechanism was progressively refined but never made independently testable, and the split-brain interpretation on which much of Eccles's cortical localization rested has not held. The work retains its place less as an active model than as the most fully developed attempt by a major philosopher and a Nobel neurophysiologist to integrate interactive dualism with empirical neuroscience.

References

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.

Campbell, D. T. (1974). "'Downward Causation' in Hierarchically Organised Biological Systems." In F. J. Ayala and T. Dobzhansky (eds.), Studies in the Philosophy of Biology: Reduction and Related Problems. London: Macmillan, 179–186.

Churchland, P. M. (1984). Matter and Consciousness: A Contemporary Introduction to the Philosophy of Mind. Cambridge, MA: MIT Press.

Eccles, J. C. (1979). The Human Mystery: The Gifford Lectures, University of Edinburgh 1977–1978. Berlin: Springer-Verlag.

Eccles, J. C. (1990). "A Unitary Hypothesis of Mind–Brain Interaction in the Cerebral Cortex." Proceedings of the Royal Society of London B 240(1299): 433–451.

Eccles, J. C. (1994). How the Self Controls Its Brain. Berlin: Springer-Verlag.

Marriott, F. H. C. (1978). Review of The Self and Its Brain, by K. R. Popper and J. C. Eccles. Leonardo 11(3): 251.

Popper, K. R. (1972). Objective Knowledge: An Evolutionary Approach. Oxford: Clarendon Press.

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.

Rosenkranz, S. (1996). "Review of How the Self Controls Its Brain, by J. C. Eccles." Sorites 9: 71–76.

Sperry, R. W. (1984). "Consciousness, Personal Identity and the Divided Brain." Neuropsychologia 22(6): 661–673.

Sperry, R. W., Zaidel, E., and Zaidel, D. (1979). "Self Recognition and Social Awareness in the Deconnected Minor Hemisphere." Neuropsychologia 17: 153–166.