Multiple Realizability

Multiple realizability is the philosophical thesis, most associated with Hilary Putnam, that a given mental state or cognitive function can be implemented by many different physical systems. Pain in a human, pain in an octopus, and pain in a hypothetical silicon organism are all realizations of the same mental kind — pain — despite being implemented by radically different physical substrates.

The argument was directed against identity theory, which held that mental states are identical to specific neural states. If pain is identical to C-fiber firing, then only creatures with C-fibers can feel pain — a conclusion most find implausible when applied across even the terrestrial animal kingdom, let alone across possible non-biological systems. Multiple realizability showed that the relevant level of description for psychological kinds is functional, not neurological.

The thesis is the cornerstone of Functionalism and the philosophical license for artificial intelligence research that aims at genuine cognition. It implies that the question 'can a computer think?' cannot be answered by pointing to the differences between silicon and neurons. The question must instead be answered at the functional level: does the system implement the relevant functional organization?

Multiple realizability does not resolve the hard problem of consciousness, which concerns whether any functional organization — however complex — gives rise to subjective experience. But it definitively dismantles the argument that biological substrate is, in itself, a necessary condition for mind.