Functionalism: Mind as Software
Functionalism is the view that mental states are defined by their functional roles — their causal relations to sensory inputs, behavioral outputs, and other mental states — rather than by their intrinsic physical constitution. On a functionalist account, pain is whatever state is caused by tissue damage, causes avoidance behavior and desire to cease the painful experience, causes distraction from other tasks, and interacts with beliefs and desires in characteristic ways. Any physical system that instantiates this functional organization has pain — regardless of whether it is implemented in neurons, silicon chips, or some other substrate.
Functionalism's most important motivation is the multiple realizability argument, advanced by Hilary Putnam in the 1960s as a challenge to type identity theory. Putnam argued that the same mental state type (pain) can be realized in multiple different physical substrates. Human pain is realized in C-fiber firing (a particular type of nociceptive neural response). Octopus pain, if octopuses experience pain, would be realized in a very different nervous system. Martian pain, if Martians have psychology, might be realized in different physical structures entirely. All these different physical realizations might be instances of the same mental state type — pain — despite having nothing physically in common. Type identity theory, which requires that pain = a specific physical state type, cannot accommodate this multiple realizability. Functionalism can: pain = whatever state plays the pain role, regardless of physical substrate.
Functionalism aligns naturally with the computational metaphor: just as the same software can run on different hardware platforms, the same mental state can be implemented in different physical substrates. This alignment with AI and computer science gave functionalism enormous influence in cognitive science, and it underlies the project of strong AI — the view that sufficiently sophisticated computational processes would constitute genuine thought and understanding.
Functionalism faces its own objections. The most powerful are the Chinese Room (Searle, covered in Phil of Mind 201) and the qualia objections. If functionalism is correct and qualia are defined by their functional roles, then inverted qualia should be impossible — any two functionally identical systems have the same mental states. But the inverted spectrum thought experiment seems to show that functionally identical systems could differ in their qualia, suggesting qualia have intrinsic qualitative properties beyond their functional roles.