Mind-Brain Identity Theory—Overview

Mind-Brain Identity Theory (also called “type physicalism” or, in its Australian form, “central-state materialism”) holds that mental states and processes are strictly identical to states and processes of the brain—not merely correlated with them, produced by them, or dependent upon them. In other words, Mind–Brain Identity Theory (MBIT) holds that mental states and processes—including phenomenal experiences—are numerically identical with physical states and processes of the brain. It does not claim merely that brain activity causes, produces, realizes, or correlates with consciousness; it claims that, where consciousness occurs, the conscious event and the relevant neural event are one and the same event described through different concepts and investigated by different methods. In its classical form, MBIT is the clearest expression of materialism about mind, but its continuing importance lies less in any simple one-to-one mapping between familiar mental kinds and fixed neural kinds than in the stronger constitutive idea that a complete theory must say what physical process an experience actually is.

Formulated between 1956 and 1959 by U.T. Place, Herbert Feigl, and J.J.C. Smart, it treats "sensations are brain processes" on the model of scientific identifications such as "lightning is an electrical discharge" or "water is H²O": one phenomenon, two routes of access, discovered empirically rather than known by analysis of meanings. The theory dominated Anglophone philosophy of mind for roughly a decade before Hilary Putnam's multiple realizability argument redirected the field toward functionalism, yet it never disappeared—surviving in token-identity and restricted type-identity forms, underwriting the neuroscientific search for neural correlates, and returning in a substantial contemporary literature that contests the multiple realizability argument directly. Its deepest significance for this Landscape of Consciousness is structural rather than historical: an identity claim of some kind appears to be a requirement on any theory of consciousness that presents itself as complete.

The Core Claim: Identity, Not Correlation

Identity is stronger than dependence. If pain is identical with neural process N, pain cannot occur on that occasion without N, because there are not two events—one mental and one neural—that might come apart. The relation resembles scientifically discovered identities such as lightning with electrical discharge or water with H₂O: the expressions differ in meaning, evidential route, and ordinary use while referring to the same underlying phenomenon. Classical identity theorists therefore rejected both Cartesian dualism and psychophysical parallelism. They also distinguished identity from semantic reduction: a person can report pain without knowing neuroscience, just as people recognized lightning before understanding electricity (Place, 1956; Feigl, 1958/1967; Smart, 1959).

The theory uses "identity" in the strict logical sense governed by Leibniz's law, not as a loose synonym for dependence or close association. It is not the claim that the brain causes the mind, nor that the mind supervenes on the brain, nor that mental states are realized by neural states. It is the claim that they are numerically one and the same, such that whatever is true of the brain process is true of the sensation—though not necessarily true of it under that description, and not necessarily known to the subject. Smart's illustration is that the professor of anatomy may be identical to the dean of the medical school: a visitor may know that the professor hiccups in lectures without knowing that the dean does (Smart, 2007). Strictly, the theory identifies states and processes rather than objects; identity theorists need not say that the mind is the brain, since we would hardly say that a mind weighs fifty ounces.

The most familiar version is type identity: each mental kind is identical with a physical kind—pain with some neural type, visual red-experience with another. Token identity is weaker: every particular mental occurrence is some physical occurrence, although instances grouped under one psychological category may belong to different physical types. “Central-state materialism” originally emphasized sensations and other inner episodes resistant to behaviorist analysis. Later causal-role theories associated with Armstrong and Lewis combined functional characterization with physical identity: folk psychology fixes the role of a mental state, and neuroscience discovers its physical occupant (Armstrong, 1968; Lewis, 1966, 1972).

Origins in Adelaide and Vienna/Minnesota

The theory has two independent points of origin. In Adelaide, U.T. Place published "Is Consciousness a Brain Process?" (Place, 1956) after extended discussion with J.J.C. Smart and C.B. Martin, arguing that identifying consciousness with brain process is a reasonable scientific hypothesis that cannot be dismissed on logical grounds. Smart, then defending a Rylean behaviorism, found Place's argument unanswerable and converted, publishing "Sensations and Brain Processes" (Smart, 1959), which became one of the most reprinted papers in analytic philosophy and gave the position its canonical form. The resulting view was known, affectionately or otherwise, as the Australian heresy.

Independently, in Minnesota, Herbert Feigl, a co-founder of the Vienna Circle, arrived at an identity thesis from the logical empiricist tradition, developing Moritz Schlick's "double-language" treatment of the psychophysical relation into the long essay "The 'Mental' and the 'Physical'" (Feigl, 1958). Anticipations exist in Carnap, Schlick, and Reichenbach, and the psychologist E.G. Boring may have been first to use the term "identity theory" (Place, 1990); older materialisms reach back to Hobbes and La Mettrie. But it was the papers of Feigl and Smart, on Jaegwon Kim's assessment, that installed the mind-body problem as a mainstream metaphysical problem of analytic philosophy and set the broadly physicalist parameters within which the subsequent debate has been conducted (Kim, 1998).

Contingent Identity and the Semantics of Reference

The first objection the founders faced was that "sensation" does not mean "brain process," so the two cannot be identical. The reply, which shaped everything after, was to separate meaning from reference. "Very bright planet seen in the morning" and "very bright planet seen in the evening" differ in sense but refer alike to Venus. Place made the point through what he called the "is" of composition, as in "this table is an old packing case": we establish that it is a table one way and that it is a packing case another way, and neither excludes the other. It follows that psychophysical identities are contingent and a posteriori—discoveries, not analytic truths. Aristotle thought the brain cooled the blood; that he was wrong was not a matter of logic.

Phenomenal Properties and the Topic-Neutral Reply

A harder objection concerns properties rather than particulars: even granting that a sensation is a brain process, are there not introspected qualities—the vivid green of an after-image—that no neural process possesses? Smart's answer was that reports of experience are topic-neutral, describable in terms that leave open whether what is occurring is physical or psychical. To report a yellowish-orange after-image is to report that something is going on like what typically goes on when one is awake, with eyes open, and an orange is illuminated in good light. Place made the complementary diagnostic move, naming as the "phenomenological fallacy" the mistake of supposing that in describing how things look or feel one is describing literal properties displayed on an internal screen. Sense data and images, on this account, are not items in the world's furniture. David Chalmers has objected that the topic-neutral formula is either too broad to be sufficient or, once narrowed to experiential states, question-begging (Chalmers, 1996), and the adequacy of the maneuver remains the crux of the dispute.

Multiple Realizability and the Turn to Functionalism

The objection that displaced the theory was Putnam's (1967). If a mental state is identical to a physical state type, then any creature in that mental state must be in that physical state type. But pain is plausibly instantiated in octopuses, mammals, and conceivably in silicon-based systems whose physical organization differs radically from ours. Pain therefore cannot be identical to any single neural type. What mental states share is functional organization, not physical constitution—a conclusion that launched functionalism and left identity theory, in its unrestricted type form, widely regarded as refuted. Two qualifications matter. First, the argument bears on type identity only: token identity, on which each particular mental event is some particular physical event, remains available and is retained even by Davidson's anomalous monism (Davidson, 1980). Second, restricted or species-relative type identities escape the argument, as in Lewis's population-relative treatment of pain (Lewis, 1980). Saul Kripke's modal argument, that "pain" and "C-fiber firing" are both rigid designators so their identity cannot be contingent, supplies a second influential line of attack (Kripke, 1980).

Contemporary Revivals and Applications

Identity theory is no longer the field's default, but reports of its death were premature, and the last three decades have produced a serious revisionist literature. Bechtel and Mundale (1999) argue that the multiple realizability argument trades on a mismatch of grain: it individuates brain states finely while individuating psychological states coarsely, whereas working neuroscience individuates both at comparable grain and routinely uses brain similarity across species to identify psychological similarity. Shapiro (2000) argues that genuine multiple realization requires differences relevant to the performance of the function, which many alleged cases do not exhibit. Polger (2009) and Polger and Shapiro (2016) have pressed both lines into a sustained defense of identity theories, and Hill (1991) defends type materialism about sensations directly. Bickle (1998) develops a "new wave" reductionism designed to survive the classical objections. Bechtel and McCauley (1999) propose a heuristic identity theory on which identity claims function less as finished metaphysics than as research strategies in cognitive neuroscience—hypothesized identities guiding the search for mechanisms and revised as the search proceeds. That last formulation captures how identity thinking actually operates in contemporary science. The neural correlates of consciousness program, though officially neutral between correlation and identity, is intelligible as a search for identities rather than mere covariation, and Integrated Information Theory states an explicit identity: an experience is identical to a maximally irreducible cause-effect structure (Tononi et al., 2016). In philosophy, David Papineau's causal argument for physicalism takes psychophysical identity as its conclusion rather than its premise, and the phenomenal concept strategy attempts to explain why identities feel counterintuitive without conceding that they are false (Papineau, 2020).

Identity as a Constraint on Every Theory of Consciousness

I/RLK take special interest in identity theory, for the Landscape, not because I subscribe to it as originally formulated, but because its way of thinking is more pervasive and more elucidating than is often realized—though perhaps not as reassuringly so as some might hope. I would generalize the classic commitment that mental states, events, and processes are identical to brain states, events, and processes. Any theory of consciousness, to be complete and sufficient, must make some kind of identity claim. Every theory that offers itself as a total explanation, necessary if not always sufficient, must be a kind of identity theory—in the strong sense, the sense in which the Morning Star and the Evening Star must both be Venus, such that if you eliminate the Morning Star you cannot have the Evening Star. Papineau makes a virtue of this necessity in his identity argument for physicalism (Papineau, 2020). It does not matter which specific materialist, physicalist, or nonphysical theory is at issue; all of them are, in essence, identity theories. Everywhere there is consciousness, some kind of consciousness identity is actually happening—always happening and never changing. Something happening or existing in every sentient creature just is consciousness.

References

Bechtel, W., & McCauley, R. N. (1999). Heuristic identity theory (or back to the future): The mind-body problem against the background of research strategies in cognitive neuroscience. In M. Hahn & S. C. Stoness (Eds.), Proceedings of the 21st Annual Meeting of the Cognitive Science Society (pp. 67–72). Mahwah, NJ: Lawrence Erlbaum.

Bechtel, W., & Mundale, J. (1999). Multiple realizability revisited: Linking cognitive and neural states. Philosophy of Science, 66(2), 175–207.

Bickle, J. (1998). Psychoneural Reduction: The New Wave. Cambridge, MA: MIT Press.

Chalmers, D. J. (1996). The Conscious Mind: In Search of a Fundamental Theory. New York: Oxford University Press.

Davidson, D. (1980). Mental events. In Essays on Actions and Events. Oxford: Clarendon Press.

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Hill, C. S. (1991). Sensations: A Defense of Type Materialism. Cambridge: Cambridge University Press.

Kim, J. (1998). Mind in a Physical World: An Essay on the Mind-Body Problem and Mental Causation. Cambridge, MA: MIT Press.

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Papineau, D. (2020). We must shed our metaphysical assumptions. Institute of Art and Ideas.

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Polger, T. W., & Shapiro, L. A. (2016). The Multiple Realization Book. Oxford: Oxford University Press.

Putnam, H. (1967). Psychological predicates. In W. H. Capitan & D. D. Merrill (Eds.), Art, Mind, and Religion (pp. 37–48). Pittsburgh: University of Pittsburgh Press.

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Smart, J. J. C. (1959). Sensations and brain processes. Philosophical Review, 68(2), 141–156.

Smart, J. J. C. (2007). The mind/brain identity theory. In E. N. Zalta (Ed.), The Stanford Encyclopedia of Philosophy. Stanford University.

Tononi, G., Boly, M., Massimini, M., & Koch, C. (2016). Integrated information theory: From consciousness to its physical substrate. Nature Reviews Neuroscience, 17(7), 450–461.