Identity theory

Identity theory (also type-type identity theory or mind-brain identity theory) is the philosophical thesis that mental states are identical to brain states — that pain, belief, desire, and other mental events just are physical events in the nervous system, described in different vocabulary. The foundational formulation belongs to J.J.C. Smart and Herbert Feigl in the 1950s, and it represents the first rigorous physicalist attempt to close the explanatory gap between mind and matter.

The theory's appeal is its parsimony: if mental states are identical to physical states, there is nothing mysterious about how mind and body interact. They are one thing, not two. The interaction problem that torments Cartesian dualism simply dissolves.

Its defeat came from multiple realizability: if pain is identical to C-fiber firing, then any creature lacking C-fibers cannot feel pain. This conclusion — that octopuses and Martians are definitionally incapable of pain because they lack our neural substrate — struck most philosophers as more implausible than identity theory itself. Hilary Putnam's argument made this rejection systematic, and functionalism rapidly displaced identity theory as the dominant physicalist position.

The displacement may have been too fast. Identity theory's critics assumed that the unit of identification must be a type (pain = C-fiber firing, in general) rather than a token (this pain = this brain event, in this system). Token identity theory — that each individual mental event is identical to some physical event, without requiring the same type of physical event across different organisms — survives multiple realizability and remains philosophically viable. It is the implicit metaphysics of most working neuroscientists who bother to have a philosophy of mind at all.

The lesson of identity theory's fall is not that functionalism is correct — it is that replacing one undefended type-identity with another (mental state = functional state) reshuffles rather than resolves the problem. The symbol grounding problem is the identity theorist's revenge: if functional descriptions must eventually bottom out in physical ones, the question of what makes a physical state the one that grounds any given meaning reasserts itself. Functionalism borrowed time from identity theory without repaying the debt.