What Is the Identity Theory?

Direct Answer

The identity theory of mind — also called the type identity theory or mind-brain identity theory — is the thesis that mental states are identical to brain states. The pain you feel when you touch a hot stove is not merely correlated with or caused by neural activity; it is that neural activity. There are not two things, a mental event and a physical event, standing in a relation of causation or correspondence; there is one thing described in two ways. "Pain" and "C-fiber stimulation" pick out the same state, just as "water" and "H2O" pick out the same substance and "lightning" and "electrical discharge" pick out the same phenomenon.

Two versions must be distinguished. Type identity theory claims that every type of mental state is identical to a type of brain state: pain in general is a particular kind of neural firing, in the way that water in general is H2O. Token identity theory makes the weaker claim that every particular mental event — this pain, now — is identical to some particular physical event, without requiring that all pains share a single physical type. Token identity is compatible with multiple realizability and is accepted by most contemporary physicalists; type identity is the stronger, more controversial thesis associated with the classic identity theory of the 1950s and 1960s.

The crucial distinction between identity and correlation matters. A correlation is a regular co-occurrence: whenever your temperature rises, the thermostat needle moves, but the needle is not the temperature. Identity theorists argue that mental-brain relations are closer than correlation. The identification of water with H2O was an empirical discovery, not a definition; likewise, they argue, the identification of pain with a neural state will be an empirical discovery of neuroscience. Once science reveals that the same kind of neural event underlies every pain, we have reason to say they are one thing, not two.

Historical Context

The identity theory emerged in the 1950s as a response to the failures of both dualism and behaviorism. Cartesian dualism — the view that mind and body are distinct substances — faced the interaction problem: how could a nonphysical mind causally influence a physical body without violating the laws of physics? Logical behaviorism avoided the problem by denying that mental states are inner events at all, but its price was high: it seemed unable to account for the qualitative character of experience, and it strained against the plain fact that our mental lives seem to be inner events with real causal powers.

Against this background, U. T. Place (1956), Herbert Feigl (1958), and J. J. C. Smart (1959) proposed the identity theory as a middle path. Mental states are inner, real, causally efficacious events — but they are not nonphysical; they are brain events. This preserved the intuition that our pains and thoughts are genuine occurrences in the world while making them fully natural. The identification, Smart argued, is analogous to familiar scientific identifications: temperature is mean molecular kinetic energy; genes are DNA sequences. Science reveals what things are, and the mind is no exception.

The theory flourished in the 1960s, promoted by philosophers such as David Armstrong and D. M. Armstrong's Australian school, which combined identity theory with a causal analysis of mental concepts. But its dominance was short-lived.

Key Arguments & Debates

The central argument for the identity theory is an inference to the best explanation. If mental states are brain states, then the causal powers of the mind — the way thoughts cause actions — are simply the causal powers of the brain, and no interaction problem arises. The theory also explains the progress of neuroscience: every advance in mapping cognition onto neural mechanisms is naturally read as revealing the nature of mental states, just as chemistry reveals the nature of water. Occam's razor favors it: it posits one kind of thing where dualism posits two.

The decisive objection came from Hilary Putnam in 1967: multiple realizability. A mental state such as pain or belief appears to be realizable in many different physical systems. Octopuses feel pain with a radically different nervous system; an alien or a robot could, in principle, have beliefs without any C-fibers. If pain is identical to C-fiber firing, then any creature without C-fibers cannot be in pain — yet it seems plainly possible that such creatures suffer. Because the identity theorist's claim is that the correlation holds for all possible cases (a nomological identity), not just actual ones, multiple realizability threatens it at the level of types. Putnam concluded that mental kinds are functional kinds, not physical kinds, and the rise of functionalism followed.

Saul Kripke added a further, more abstract objection. If "pain" and "C-fiber firing" are identical, the identity is necessary — they are the same thing in all possible worlds. But we can conceive of pain without C-fiber firing (a creature that feels pain but lacks the fibers) and of C-fiber firing without pain (a brain in that state with no experience). Kripke argued that for genuine identities, like water and H2O, the conceivability of divergence is explained away by ignorance of underlying structure; but in the mental case, there is no such gap — experience is its own essence, so conceivability here is evidence of possibility. This remains a live challenge.

Defenders of identity respond that multiple realizability only refutes type identity, not the token identity that most physicalists actually need, and that Kripke's argument presupposes a Cartesian conception of phenomenal properties that physicalists reject.

Contemporary Relevance

In contemporary cognitive neuroscience, some form of identity thesis is the working assumption: the search for the neural correlates of consciousness treats consciousness as, at minimum, realized in and perhaps identical with brain processes. Patricia Churchland and other neurophilosophers defend an updated identity theory, arguing that as neuroscience matures, mental kinds will be reduced to neural kinds just as thermodynamic kinds were reduced to statistical-mechanical ones. The debate has also re-emerged in the philosophy of artificial intelligence: if mentality is type-identical to biological brain states, then silicon systems that only simulate cognition are not genuinely conscious; if mentality is functional, they might be.

At the same time, the persistence of the hard problem — why brain processes are accompanied by experience at all — keeps the identity theory contested. For those who think the explanatory gap is real, identity is an article of faith rather than a discovery. For those who think the gap is an illusion, identity theory is simply what a mature science of mind will reveal. The question of whether the mind is the brain remains, in this sense, the defining question of the field.

Related Concepts

• Behaviorism: the view identity theory replaced; mental states as dispositions rather than inner events.
• Functionalism: the multiple-realizability-based alternative that succeeded it.
• Multiple realizability: the key objection to type identity.
• Supervenience: the weaker dependence relation most physicalists now accept.
• Phenomenal consciousness: the aspect of mind identity theory most challenged to explain.
• Emergentism: the view that mental properties are new properties arising from but not identical to physical ones.

Further Learning

• Stanford Encyclopedia of Philosophy: The Mind/Brain Identity Theory
• Internet Encyclopedia of Philosophy: Mind/Brain Identity Theory
• U. T. Place, "Is Consciousness a Brain Process?," British Journal of Psychology 47 (1956).
• J. J. C. Smart, "Sensations and Brain Processes," Philosophical Review 68 (1959).