Mind as Brain, Described Twice

The identity theory that anchors physicalism holds that a mental state, the sensation of pain, for instance, simply is a physical state of the brain, a particular pattern of neural firing, in exactly the way that lightning simply is electrical discharge and water simply is H2O: not correlated with the physical state, not caused by it, but identical to it, two names for one thing discovered by empirical investigation rather than by definition.

J.J.C. Smart argued this in the mid-twentieth century partly on grounds of parsimony, since positing an additional non-physical fact over and above the brain state seemed to him to multiply entities without genuine explanatory payoff once a mature neuroscience could correlate every mental event with a physical one. The theory's chief technical difficulty is the so-called explanatory gap: even granting that pain and a certain neural firing pattern always occur together, nothing in the physical description of the firing pattern seems to logically entail what the pain feels like from the inside, a worry pressed hardest by thought experiments about a color scientist who knows every physical fact about color vision but has never seen red. Functionalism arose partly to sidestep the gap by defining mental states by their causal role rather than their physical composition, while remaining broadly physicalist about what, in the end, realizes that role.