Does Mind-Brain Type Identity Theory Give a Successful Account of Mental States?
MBTIT does not give a successful account of mental states. It survives Descartes' conceivability argument via the water/H₂O analogy, and survives Jackson's knowledge argument via the new knowledge/old fact response. But the problem of multiple realisability has no clear answer within type identity theory — it defeats MBTIT on its own empirical terms. 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. 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 can successfully respond to two powerful dualist arguments — Descartes' conceivability argument and Jackson's knowledge argument — but has no adequate reply 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: Descartes' Conceivability Argument
- Descartes' conceivability argument has initial appeal — it does seem possible to imagine a disembodied mind in a way that it does not seem possible to imagine water that is not H₂O. The argument captures a genuine intuition about the apparent difference between mental and physical - However the water/H₂O analogy is decisive. If conceivability of separation were sufficient for non-identity, established scientific reductions would be undermined. The argument proves too much and must therefore be rejected. MBTIT survives this challenge - This does not mean the identity theorist has fully resolved the tension — the conceivability of disembodied minds may track something philosophically important about the nature of consciousness. But it does mean that the conceivability argument alone cannot defeat MBTIT
Descartes: Descartes' Conceivability Argument
- Descartes argues that mind and brain are distinct substances on the grounds that they can be conceived of separately. He can clearly and distinctly conceive of himself as a thinking thing existing without any body, and he cannot doubt his own existence as a mind even while doubting the existence of everything physical. Since he can conceive of mind without brain, they must be distinct — and if they are distinct, MBTIT, which claims they are identical, is false - The underlying principle is that if two things can be conceived of as existing independently of each other, they cannot be identical. Identity is necessary — a thing cannot be separated from itself — so if separation is conceivable, identity is ruled out
: MBTIT's Response — The Water/H₂O Analogy
- The identity theorist replies that Descartes' argument proves too much. If conceivability of separation were sufficient to establish non-identity, we could prove that water is not H₂O or that heat is not molecular motion in exactly the same way. Conceiving of a world with heat but no molecular motion seems just as possible as conceiving of a world with minds but no brains. But the fact that we can do this does not show that heat is not molecular motion — we now know empirically that it is. The conceivability argument must therefore go wrong somewhere, since it would rule out established scientific identities - Furthermore, once we properly understand how the brain works, we may come to see that minds cannot exist without brains. Unless Descartes can show a principled difference between the mind/brain case and the water/H₂O case, his conceivability argument does not establish that mind and brain are distinct
Section 2: Jackson's Knowledge Argument
- The knowledge argument is more powerful than the conceivability argument because it targets something specific and difficult — the subjective, first-person character of experience — rather than relying on a general principle about conceivability. The intuition that Mary learns something genuinely new is hard to dismiss - However the new knowledge/old fact response is the most philosophically credible reply. The Clark Kent analogy shows that learning an old fact from a new perspective can feel like a genuine discovery without there being any new non-physical fact. It is at least possible that this is what happens when Mary sees red - The response is not fully decisive — one could argue that the essentially subjective character of qualia means they can never be captured by objective physical science, regardless of perspective. But it is enough to show that the knowledge argument does not conclusively refute MBTIT
Jackson: Jackson's Knowledge Argument
- Frank Jackson's knowledge argument targets the claim that all facts about mental states are physical facts. It proceeds through the thought experiment of Mary, 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 physicalism is false and there are non-physical facts about consciousness that MBTIT cannot accommodate - The argument targets qualia — 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 — New Knowledge, Old Fact
- The best response is the new knowledge/old fact reply. Although Mary does gain new propositional knowledge when she sees red, this is knowledge of a physical fact she already knew — just encountered from a new perspective. An analogy: I might know that Superman is brave without knowing that Clark Kent is brave, because I do not know that Superman is Clark Kent. When I discover that Clark is brave, it feels like a genuine new discovery, yet I am learning a fact I already knew in a different way - The same may apply to Mary — she gains a new phenomenal concept that allows her to know the same physical fact from a first-person perspective. The new knowledge is not evidence of a new non-physical fact
Section 3: The Problem of Multiple Realisability
- The multiple realisability problem is the strongest objection to MBTIT because it is not a philosophical thought experiment but an empirical finding. MBTIT stakes its credibility on being an empirical hypothesis, and the empirical evidence of brain plasticity and cross-species mental states directly falsifies the type identity claim. There is no clear way for MBTIT to reply - Token identity theory avoids the problem but at a significant cost. Critics argue it is too liberal to count as a genuine identity theory — if each token mental state can be identical to a different type of brain state, the identity claim becomes so weak that it is difficult to see what work it is doing. Someone who said that water is sometimes H₂O and sometimes something else would be in danger of speaking nonsense. The same worry applies here - Nevertheless token identity theory retains the core physicalist insight — that mental states are identical to brain states — while accommodating the empirical evidence. It is a more defensible position than MBTIT, even if it faces difficulties of its own
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 - An octopus can experience pain, yet it does not have c-fibres. If pain were identical to c-fibre firing, an octopus could not be in pain — yet the behavioural evidence strongly suggests it can. Similarly, the plasticity of the human brain shows that if c-fibres are damaged, other neural structures can take over their functional role, meaning pain can be realised by different brain states even within a single individual over time - Putnam strengthens the objection by moving beyond empirical examples to a logically possible case. We can conceive of an alien with a completely different physical anatomy — no c-fibres, no human neural architecture — 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 is more powerful than either of the dualist arguments in Sections 1 and 2 because it attacks MBTIT on its own terms. MBTIT presents itself as an empirical hypothesis confirmed by neuroscience — but the empirical discoveries of neuroscience, particularly brain plasticity and cross-species studies, are precisely what undermines it
: Token Identity Theory as a Partial Escape
- Identity theorists can respond by moving from type to token identity theory. Rather than claiming that all tokens of a mental type are identical to tokens of the same physical type, token identity theory claims only that each individual token of a mental state is identical to some brain state token — which might be a different type of brain state in different individuals or species. Pain in me might be c-fibre firing; pain in an octopus might be a completely different neural event. The type identity is abandoned but individual identity is preserved - This move avoids the multiple realisability problem because it no longer claims that pain must always be realised by the same type of physical state
- MBTIT survives Descartes' conceivability argument: the water/H₂O analogy shows that conceivability of separation does not entail non-identity, and the argument proves too much by threatening established scientific reductions - MBTIT survives Jackson's knowledge argument: the new knowledge/old fact response shows that Mary's discovery when seeing red may be a new perspective on a physical fact she already knew, not evidence of a non-physical fact. The Clark Kent analogy supports this - MBTIT cannot survive the problem of multiple realisability. The empirical evidence of brain plasticity and cross-species mental states directly falsifies the type identity claim on its own terms. Token identity theory avoids this by abandoning type identity while preserving individual identity — but faces questions about its coherence as an identity theory - MBTIT therefore does not give a successful account of mental states. Mind-brain token identity theory, which retains the core physicalist commitment while accommodating multiple realisability, may do better