Mind-brain identity theory
Explanation
In the 1950s, Australians U. T. Place and J. J. C. Smart, together with Englishman Herbert Feigl, proposed a clean solution to the mind-body problem: mental states are literally identical to brain states. It is not that the mind emerges from the brain or correlates with it: it is the same thing, described in two ways. Just as lightning is an electrical discharge, or as water is H₂O.
The argument relies on an analogy with previous scientific discoveries. Earlier no one knew that water was H₂O; the two descriptions were arrived at independently. When chemistry unravelled the molecular structure, we discovered they referred to the same thing. Analogously, today we speak of pain (in a psychological vocabulary) and of patterns of neural activity (in a neuroscientific vocabulary), but as the science matures we will discover that pain IS such a pattern.
This is a type identity: every type of mental state corresponds to a specific type of brain state. If two people feel pain, they must have the same (or at least an equivalent type of) neural activity. This is a strong empirical claim, susceptible to verification or refutation by neuroscience.
The virtues of the proposal are considerable: it is monist (a single substance, matter), it requires no mysterious substances, it fits the success of neuroscience, and it respects physical causation. If the mind is the brain, there is no need to explain how two distinct substances interact: they are a single thing in action.
The main objection came from functionalism: multiple realisability. If mental states are identical to specific brain types, then creatures with different brains (aliens with exotic chemistry, robots with silicon) could not have the same mental states as us. But it seems conceivable that a Martian with a very different brain could feel pain; what would matter would not be the substrate but the function.
The theory evolved into more modest versions, such as token identity: each particular mental state is identical to some particular brain state, without claiming that all pains share the same type of neural activity. In this moderated version, the theory remains defensible and holds a central place in the contemporary philosophical debate on consciousness.
Strengths
- Parsimonious solution to the mind-body problem.
- Compatible with the development of neuroscience.
- Realist about mental states, not eliminativist.
- Clear model of how other theoretical identities work in science.
Main critiques
- Multiple realisability argument (Putnam): the same mental state can have different physical substrates.
- Does not explain qualia: why does the firing of C-fibres feel like that?
- Inverted spectrum and analogous arguments: type identity is contingent, but then what justifies it?
- Knowledge argument (Mary's Room): learning a new quale adds information.