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 (henceforth MBTIT) claims that all mental states are identical to brain states, but that the words 'mental state' and 'brain state' are not synonymous. It thus gives an ontological reduction, rather than an analytic reduction, of mental states to brain states. - Crucially, this distinction tracks Frege's sense/reference distinction: a mental term and a neural term may have different senses — that is, different modes of presentation — 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, it also fits naturally with the causal closure principle. - 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
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 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, namely what it is like to see red (the qualia of seeing red). - If physicalism were true, she could not have learnt any new facts as she already knew all physical facts. 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. - The **reference** of a term is the thing it picks out in the world. The **sense** is the mode of presentation of that reference — the way the reference is given to us, the cognitive route by which we get at it. - 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: Kripke's Argument Against Identity Theory
- Kripke argues that all true identity statements are necessarily true: if a is identical to b, they cannot come apart in any possible world. - Since 'pain' and 'c-fibres firing' are rigid designators, meaning that they refer to the same thing, they do so in every possible world. But we can coherently conceive of pain without c-fibres firing — an alien or octopus might experience pain without c-fibres, and c-fibres might fire without producing pain. - For rigid designators, this conceivability is a reliable guide to genuine metaphysical possibility. - So pain and c-fibres firing can come apart, and are therefore not identical.
: MBTIT's Response
- Attack P2: The conceivability of pain without c-fibres may reflect our epistemic limitations rather than genuine metaphysical possibility. - One might argue that we can conceive of water without H₂O, for example someone ignorant of chemistry could do so. However, this is not possible. If we attempt to conceive of water (a blue liquid that we can swim in and drink) without H₂O what we really conceive of is a different chemical that appears the same as water, call that chemical XYZ. - In the same way, when we attempt to conceive of pain without c-fibres firing, as Kripke argues we can, we do not actually conceive of pain, we conceive of a pain substitute.
Kripke: Why this response is strained — Kripke's structural disanalogy
- Kripke's argument is designed to block this move by identifying a structural disanalogy between natural kinds and phenomenal kinds. - Natural kinds like water have a hidden essence (H₂O) distinct from their appearance (clear drinkable liquid). This is what makes the XYZ move available: something can share water's appearance without sharing its essence, and that something is the water-substitute we are really conceiving of. - Phenomenal kinds like pain have no such distinction. Pain's essence just **is** its felt quality — there is no hidden nature behind the feeling. So when we conceive of pain without c-fibres, we cannot be conceiving of 'something with pain's appearance but not its essence,' because for pain, appearance and essence are the same. Anything with the appearance of pain is pain. The XYZ move has nothing to grip.
Section 3: The Problem of Multiple Realisability
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. - But the same mental type appears to be realisable by different physical types. An octopus shows pain behaviour despite having no c-fibres, and within humans, brain plasticity means that when c-fibres are damaged, other neural structures can take over the pain role. The type identity fails both across species and within individuals. - This suggests that what unifies different instances of pain is not their physical realisation but their causal/functional role — a result that points toward functionalism rather than MBTIT.
: 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.
: Objection to token identity theory
- 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 does not give a successful account of mental states. It survives Jackson cleanly via the phenomenal concepts reply and reaches an impasse with Kripke, but it cannot survive multiple realisability — the objection attacks MBTIT on the empirical terms on which it staked its credibility. - The move to token identity does not rescue the theory: without type identities, it cannot explain type unity without appealing to functional role, which is functionalism's answer, not MBTIT's. Identity theory fails in both its type and token forms to give a successful account of mental states.