Does Mind-Brain Type Identity Theory Give a Successful Account of Mental States?
MBTIT does not give a successful account of mental states. It cleanly survives Jackson's knowledge argument via the phenomenal concepts reply, and strainedly survives Kripke's modal argument, but has no adequate response to the problem of multiple realisability. Mind-brain token identity theory may do better.
- Mind-brain type identity theory holds that types of mental states are numerically identical with types of brain states — that mental states are nothing over and above physical processes in the brain. This is an ontological reduction, not an analytic one: the claim is not that 'pain' means the same as 'c-fibres firing' but that pain just is c-fibres firing, in the same way that water just is H₂O. Crucially, this distinction tracks Frege's sense/reference distinction: a mental term and a neural term may have different senses — different meanings — while referring to the same thing. 'Pain' and 'c-fibres firing' have different senses but, if MBTIT is true, the same reference. The identity is an empirical hypothesis to be confirmed by neuroscience, not a conceptual truth discoverable by analysis. - The theory has significant strengths. It is supported by the correlation between mental states and brain states discovered by neuroscience — when subjects report pain, c-fibres are observed to fire. It also fits naturally with the causal closure principle: every physical event has a complete physical cause, which means that if mental states are to have causal effects on the body, they must themselves be physical. And it avoids the interaction problem that afflicts dualism — if mind and brain are identical, there is no mystery about how they causally interact. - I will argue that MBTIT cleanly survives Jackson's knowledge argument via the phenomenal concepts reply, and strainedly survives Kripke's modal argument, but has no adequate response to the problem of multiple realisability. I will conclude that MBTIT does not give a successful account of mental states, although mind-brain token identity theory may do better.
Section 1: Jackson's Knowledge Argument
- The knowledge argument has genuine force because it targets something experientially vivid — the subjective character of conscious experience. The intuition that Mary learns something genuinely new is hard to dismiss, precisely because qualia feel so categorically different from objective physical descriptions. - The phenomenal concepts reply is the most philosophically credible response available. Its strength lies in the precision of the Fregean framework: by distinguishing sense from reference, MBTIT can accommodate the appearance of new knowledge — Mary genuinely acquires a new concept — without conceding a new non-physical fact. Crucially, the Fregean framework was built into the statement of MBTIT at the outset, which gives the reply genuine theoretical unity rather than the appearance of an ad hoc escape. - However the reply is not fully decisive. The deeper problem is that Frege's framework shows how two descriptions can share a reference — but does not by itself show that the reference is a physical thing. If phenomenal concepts can only be acquired through direct experience, it remains an open question whether the fact they refer to is physical at all. The reply shows Mary's new knowledge need not imply a new fact, but it does not conclusively close off the possibility that it does.
Jackson: Jackson's Knowledge Argument
- Frank Jackson's knowledge argument targets the claim that all facts about mental states are physical facts. Mary is a neuroscientist who knows every physical fact about colour vision — every wavelength, every neural process, every functional role — but has spent her entire life in a black-and-white room and has never seen red. - When Mary leaves the room and sees red for the first time, she learns something new — what it is like to see red. If physicalism were true, she already knew everything — so she cannot learn anything new. But she does learn something new. Therefore there are non-physical facts about consciousness — qualia — that MBTIT cannot accommodate. - The argument targets the subjective, first-person character of conscious experience. Even complete physical knowledge, Jackson argues, leaves out what it is like to have an experience.
: MBTIT's Response — Phenomenal Concepts Reply
- The phenomenal concepts reply draws directly on the Fregean sense/reference distinction introduced in the account of MBTIT. Before leaving the room, Mary knew the physical fact about colour perception under a physical description — she had a physical sense for that reference. When she sees red, she acquires a new phenomenal concept — a new sense — for the same physical referent. The new knowledge is a new mode of presentation, not a new non-physical fact. - This is illustrated by the Clark Kent case. I might know that Superman is brave without knowing that Clark Kent is brave, because I do not yet know that Superman is Clark Kent. When I discover Clark Kent is brave, it feels like a genuine new discovery — yet I am acquiring a new sense for a fact I already knew under a different sense. The reference — the fact about bravery — is unchanged. Mary's situation is analogous: she acquires a phenomenal concept that gives her a new sense for the same physical reference she already knew under a neural description.
Section 2: Kripke's Argument Against Identity Theory
- Kripke's argument is more technically precise than standard conceivability arguments against MBTIT because it exploits the identity theorist's own commitment to necessary a posteriori identities. It cannot be dismissed by pointing to scientific precedents — Kripke's analysis is designed specifically to show that the pain case is structurally different from those precedents. - The response — that our modal intuitions are epistemically unreliable — is not hopeless. The history of science does show that apparent conceivabilities dissolve with better theory. But the response is genuinely strained: the identity theorist concedes that they cannot yet explain why the conceivability intuition is misleading in the way they can explain it for water=H₂O, and can only hope that a future explanation exists. This is a significant dialectical concession.
Kripke: Kripke's Argument Against Identity Theory
- The identity theorist claims that pain is identical to c-fibres firing. Kripke argues this creates an immediate problem by examining what kind of identity claim this would have to be. The necessity of identity holds that if two things are genuinely identical — if a really is b — then they cannot be separated in any possible world, because a just is b. There is no possible world where a exists without b, since they are the same thing. Identity is therefore always necessary, never contingent. - This applies directly to MBTIT. 'Pain' and 'c-fibres firing' are both rigid designators — they are not descriptions that pick out different things in different possible worlds, but terms that refer directly and fixedly to the same thing in every possible world, if they refer to the same thing at all. So if pain is identical to c-fibres firing, this identity holds necessarily — in every possible world, wherever there is pain, there is c-fibre firing, and wherever there is c-fibre firing, there is pain. - But this is precisely where the problem arises. We can coherently conceive of a world in which someone is in pain but has no c-fibres at all — an octopus, an alien, or a future human with radically different neurology. We can equally conceive of a world in which c-fibres fire but produce no pain — perhaps through some neural suppression mechanism. If these worlds are genuinely possible, then pain and c-fibres firing can come apart — they are not identical in every possible world — and therefore, by the necessity of identity, they are not identical at all. MBTIT is false. - Kripke anticipates the obvious response: that we can also seem to conceive of water without H₂O, yet water just is H₂O. His reply is that the two cases are disanalogous. When we imagine 'water' without H₂O, we are really imagining a different substance — a watery-looking liquid, XYZ — that merely resembles water. We are not imagining water itself coming apart from H₂O. So the conceivability of 'water' without H₂O can be explained away: it is conceivability of a water-substitute, not water. But when we imagine pain without c-fibres firing, we are imagining pain itself — the actual sensation — occurring without c-fibres. There is no equivalent 'pain-substitute' move available. The conceivability is genuine, not explicable away, and it directly entails that pain and c-fibres firing are not necessarily identical — and therefore not identical at all.
: MBTIT's Response — Modal Intuitions are Unreliable
- Attack P2: The conceivability of pain without c-fibres may reflect our epistemic limitations rather than genuine metaphysical possibility. Just as someone ignorant of chemistry could conceive of water without H₂O before the identity was discovered, our ability to imagine pain without c-fibres may simply reflect our incomplete theoretical understanding of the mind-brain relationship, not a genuine possible world in which they come apart. - The identity theorist must argue that our modal intuitions about pain are unreliable. We do not yet have the theoretical framework that would make the mind-brain identity feel necessary, just as pre-scientific thinkers lacked the framework that would make water=H₂O feel necessary. As neuroscience advances and the identity is better understood, the conceivability of pain without c-fibres may dissolve — just as the conceivability of water without H₂O dissolved once the chemistry was understood. - However, Kripke's argument is specifically designed to block this move. His point is not merely that pain and c-fibres seem separable — it is that the pain case is structurally disanalogous to water=H₂O in a principled way. In the water case, we can identify what we are imagining when we imagine 'water' without H₂O: we are imagining a water-substitute, XYZ. In the pain case, there is nothing analogous to identify. We are imagining pain itself. The identity theorist cannot simply assert that our intuitions are unreliable — they must show where the disanalogy goes wrong, and Kripke's argument makes that very difficult to do.
Section 3: The Problem of Multiple Realisability
- This objection is more powerful than either the knowledge argument or Kripke's argument because it is not a philosophical thought experiment or a modal intuition — it is an empirical finding. MBTIT stakes its credibility on being an empirical hypothesis confirmed by neuroscience, and the empirical evidence of brain plasticity and cross-species mental states directly falsifies the type identity claim on its own terms. - The fine-grained type identity response is ad hoc and risks collapsing MBTIT into functionalism. Token identity theory preserves the physicalist core but abandons the type identity claim that defines MBTIT — and without type identities, it cannot explain type unity without appealing to functional role, which is functionalism's answer, not MBTIT's.
Putnam: The Problem of Multiple Realisability
- MBTIT claims that each type of mental state is identical to a specific type of brain state — pain is c-fibres firing, always and everywhere. The problem is that the same type of mental state appears to be realisable by different physical states in different creatures and within the same individual over time. - An octopus can experience pain — the behavioural evidence strongly suggests this — yet it does not have c-fibres. If pain were identical to c-fibre firing, an octopus could not be in pain. Similarly, the plasticity of the human brain shows that if c-fibres are damaged, other neural structures can take over their functional role, producing pain via a different physical realiser. The type identity fails even within a single organism. - Putnam strengthens the objection by moving beyond empirical examples: we can conceive of an alien with a completely different physical anatomy that nonetheless experiences pain. If pain can in principle be realised by any physical structure that performs the right functional role, pain cannot be identical to any particular type of physical state. - This objection points toward functionalism as the more defensible theory: what characterises mental states is not their physical realiser but their causal/functional role. Pain is whatever state plays the pain role — causing avoidance behaviour, distress, motivating withdrawal — regardless of whether that role is played by c-fibres, octopus neurons, or silicon circuits.
: MBTIT's Responses — Fine-Grained Type Identity and Token Identity Theory
- Fine-grained type identity: redefine the relevant physical type at a higher level of abstraction — perhaps pain is identical not to 'c-fibres firing' specifically but to some more abstract neural property that is instantiated across different physical realisers. Problem: this response is ad hoc. It rescues MBTIT only by moving the goalposts — the type identity becomes so abstract that it is difficult to distinguish from a functional role, collapsing MBTIT into the very theory the objection favours. - Token identity theory: abandon type identity while preserving individual token identity. Each token of a mental state is identical to some brain state token, but different tokens of the same mental type may be identical to different physical types. Pain in me is c-fibres firing; pain in an octopus is a different neural event. This accommodates multiple realisability but at a significant cost. - Token identity theory is too permissive to count as a genuine identity theory. Without type identities, it cannot explain what makes pain in a human and pain in an octopus both instances of the same mental type — what unifies the tokens as belonging to the same kind. The theory abandons the claim that gave MBTIT its explanatory force. This is precisely the gap functionalism fills: what unifies all tokens of pain is the shared causal/functional role they play, not the physical type they instantiate. Token identity theory, in abandoning type identity, finds itself requiring functionalism to do the explanatory work it was supposed to do itself.
- MBTIT cleanly survives Jackson's knowledge argument: the Fregean phenomenal concepts reply shows that Mary's new knowledge can be understood as a new sense for a physical referent she already knew, rather than evidence of a non-physical fact. The reply is contested but precise and theoretically integrated. - MBTIT strainedly survives Kripke's argument: the identity theorist can assert that modal intuitions about pain are epistemically unreliable, but cannot yet demonstrate why the pain case is analogous to water=H₂O in the way Kripke's own analysis denies. The theory survives but makes a promissory note it cannot cash. - MBTIT cannot survive the multiple realisability problem. The empirical evidence directly falsifies the type identity claim on MBTIT's own terms. Neither fine-grained type identity nor token identity theory provides an adequate response without abandoning or transforming the theory beyond recognition. - It should be noted that MBTIT may give a more successful account of some mental state types than others: pain, as a clearly physical and neurally trackable state, may be more amenable to type identity than propositional attitudes or imaginings, which seem more resistant to reduction. But this partial concession does not rescue the general theory. - MBTIT does not give a successful account of mental states. Mind-brain token identity theory retains the physicalist commitment while accommodating multiple realisability — but without type identities it cannot explain what makes different tokens of the same mental state the same type, and requires functionalism to fill this explanatory gap.