"Yesterday I discussed John Searle's Chinese Room argument, which
denies that computers are capable of thought, and that computer
functionalism is a valid description of the mind ("Is Your Brain a Computer?"). Reader may be interested to read a little more about the idea of functionalism." says Michael Egnor.
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With the eclipse of behaviorism and identity theory in philosophy of the mind, functionalism has become perhaps the dominant perspective in philosophy of the mind for philosophers and neuroscientists with a materialist perspective. Functionalism is the view that mental states are constituted entirely by their functional roles, rather than by their strict behavioral consequences, or by their physical correlates, or by their immaterial natures.
For example, a functionalist would define the mental state of experiencing pain as the functional connection between the inflammation of the nerve in your tooth and your moaning that your tooth hurts. A functionalist would define the mental state of believing that it's raining as the connection between your perception that it's raining outside and your act of unfolding your umbrella.
The functionalist perspective is often expressed by quip "The mind is what the brain does."
Because in functionalism mental states are functional relationships between physical events, inherent to most variants of functionalism is the theory of multiple realizability. Multiple realizability is the idea that mental states can arise in any system that carries out certain functions, whether or not the system is biological. Multiple realizability posits, for example, the computers can have mental states if functional relationships between electromechanical events in computers are sufficiently similar to functional relationships between electrochemical events in brains.
For example, if the electrical output of a neural network is similar in relevant ways (e.g. voltage, current, frequency, etc.) to the electrical output of the circuit in a computer, the computer will have the same mental state as the person with a neural network in his brain. Multiple realizability is the idea that any system, with the proper functional relationships, can have mental states regardless of the physical material out of which the system is made. Multiple realizability is the metaphysical basis for the theory of strong artificial intelligence.
Functionalism has gained popularity in the largely materialist philosophical community, as well as among neuroscientists who are mostly materialists, for several reasons.
Source: Discovery Institute