Multiple realisability
Overview
Multiple realisability is the thesis, introduced by Hilary Putnam in 1967, that the same mental state can be realised by different physical states. Pain in a human, an octopus and a (hypothetical) Martian need not share any physical property, only a functional one: the role the state plays in the system that has it. The argument was aimed at identity theory, which holds that each type of mental state simply is one type of brain state, and it is the main support for functionalism.
Notes
- The examples are not hypothetical. Octopuses, corvids and mammals build nervous systems differently and from different cell types, and all show learning, play and pain behaviour. Within one species no two brains are wired alike, and a damaged brain reroutes a function through tissue that previously did something else.
- My position (Substrate Independence) extends the thesis past biology: if the same function can be realised in two kinds of neuron, there is no principled reason it cannot be realised in something that is not a neuron. The objection that silicon is "the wrong kind of thing" has to say what kind of thing is the right kind, and why.
- The thesis cuts against strong reduction but not against physicalism. Every realisation is physical; what it denies is that there is only one.