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Token-identity theory states that each individual mental event is identical to some physical (neural) event, even if different mental states can be realized by different physical states. This avoids the strict type-identity requirement while maintaining physicalist monism about particular events.
Understand the relationship between type and token identity. Use examples of multiple realizability to see why token identity is more plausible than type identity.
To understand token-identity theory, you need to hold two things you have already learned in productive tension. From identity theory, you know the basic physicalist move: mental states *are* brain states — pain is C-fiber firing, belief is some neural configuration. From multiple realizability, you know the most powerful objection: the same mental state can be physically realized in radically different ways across different organisms, making any one-to-one mapping between mental and physical types implausible.
Token-identity theory is the position that emerges when you take the force of that objection seriously while refusing to abandon physicalism. The key distinction is between types and tokens. A *type* is a kind or category — "pain" is a mental type, "C-fiber firing" is a physical type. A *token* is a particular instance — *your* pain right now is a token, *this* firing of your C-fibers at this moment is also a token. Type-identity theory identifies types: pain *as a kind* = C-fiber firing *as a kind*. Token-identity theory identifies only tokens: *this* pain token = *this* neural event token. No commitment is made that every pain token across every creature must be the same physical type.
Think of it this way: the word "bank" appearing in this sentence and the word "bank" appearing in another sentence are two tokens of the same type. But suppose you have a very different claim: each individual word token in any text is identical to some physical ink pattern, without claiming all tokens of the word "bank" are physically the same. That is the logical structure of token identity — identity claims about particulars, not universals. Donald Davidson's anomalous monism is the best-known version: every mental event token is identical to some physical event token, but mental event types do not correspond to physical event types under any lawlike regularities. Mental descriptions and physical descriptions are two irreducibly different *ways of describing* the very same events.
This gives token-identity theory a distinctive profile of advantages and problems. It handles multiple realizability cleanly: your pain and an octopus's pain can both be real mental events, each identical to some neural event, without there being any single physical type that "pain" reduces to. It preserves physicalism — there are no non-physical substances or properties floating around; every mental event just is a physical event. But it generates the mental causation problem in a new form: if mental event tokens are identical to physical event tokens, do mental *descriptions* add any causal explanatory power? When we say your belief caused your action, is "belief" doing any real causal work, or is the causal story told entirely in physical terms, with "belief" being a redescription? Davidson's answer was that mental descriptions are indispensable for prediction even if causation is always implemented physically — but this remains contested. Token identity buys physicalism at the price of making the mental causally relevant only in a qualified sense, which is the trade-off you will want to examine carefully.
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