Functionalism is a materialist philosophy of mind. It is due largely to the influential work of Hilary Putnam.
As a materialist, the functionalist is committed to the supervenience thesis (see earlier lecture). But the functionalist's intention is to be uncommitted to any thesis of reducibility—indeed, to deny it. The functionalist regards both ordinary and theoretical talk about the mental as irreducible to talk about the physical. The mentalistic level of description and explanation of human behavior is sui generis and indispensable.
Functionalism can be understood as a natural development of logical behaviorism, aimed at remedying the latter theory's shortcomings. (See the relevant discussion of the Problem of Holism towards the end of the our account of Ryle's logical behaviorism.) To anticipate: for the functionalist, to be in a mental state is to occupy a certain 'logical state' in the execution of the 'program', or 'software' that is running on the brain as 'hardware'. The functionalist does not follow the logical behaviorist's strategy of trying to state, locally as it were, the behavioral dispositions in which occupancy of such a state consists. Instead, the functionalist contents himself with specifying the complicated interconnections among such states, allowing them to interact in multifarious ways in mediating between sensory input and motor output.
This raises another difference between logical behaviorism and functionalism. The logical behaviorist is content to see the mind/brain as a black box, whose internal structure—either physical or functional—is irrelevant to his conception of what the occupancy of a mental state consists in. For the logical behaviorist, it does not matter how the dispositions to behavior might arise—only that they be in place. By contrast, the functionalist needs to break into the functional structure or organization of the mind/brain, in order to attain a deeper characterization of mental states. Our ultimate understanding of that functional structure or organization might well involve understanding exactly how our grey matter works. That is, we might need to learn everything we can about neurons, axons, dendrites, synaptic thresholds, neurochemicals, etc. But we shall be doing this only with an eye to abstracting the crucial functional structure and organization.
The functionalist's emphasis on abstraction here marks an importance difference from the biological essentialist. For the biological essentialist, the human brain 'secretes consciousness' because of its essential biological make-up. These neurological structures, these neurochemical concentrations, these foldings and fissures of the cortex, etc. are, on this essentialist view, what give rise to consciousness. The biological essentialist maintains that an artificial behavioral duplicate of a human being—an android, say, like those in the movie Blade Runner, or like Arnold Schwarzenegger's Terminator, or like Nanette Newman's character in The Stepford Wives, or like the Yul Brunner character in Westworld— would lack consciousness, for want of the right kind of brain matter.
This is where the functionalist begs to differ. As the label suggests, all that should count is the function of the physical state one is in; not the physical nature of that state. Thus the functionalist would ascribe a mental make-up (even: consciousness) to any behavioral duplicate of a human being, be it an artifact with a 'brain' of silicon chips, or an extraterrestrial creature with a radically different biology. The technical term used in this connection is multiple realizability. Just as the same software can run on radically different kinds of harware (think of Word for the Mac v. Word for an IBM-clone PC), so too the same 'mentality' can be realized on radically different kinds of meatware or hardware.
Nor should too much emphasis be placed on exact duplication. There is no reason, on the functionalist's view, to refrain from saying that chimpanzees are conscious, or that extraterrestrials with radically different sensory modalities and behavioral repertoires are. Presumably these cases would transcend some minimal 'threshold' of complication in internal organization to count as conscious (or as having beliefs and desires, if we wish to stress the psychological rather than the phenomenological).
The functionalist conceives of one's mind as a program. A program specifies how the system is to make transitions among different (logical) states, in response to current monitored inputs; and also how it is to create outputs. In the simplest case, these transitions will be deterministic, in that everything about one's immediate next state, in response to current input, will be exactly specified by the program. In the more general case, the transitions will be probabilistic, allowing for a range of possible successor states, each with a different probability of being the state actually entered into. Unless the functionalist opts for the probabilistic model of 'mind-as-program', she will face difficulties with the problem of free will.
The most general analysis of computational programs is due to Alan Turing, the founding father of computer science. The concept of a Turing Machine is named after him.
The next lecture is devoted to an explanation of the concept of a Turing Machine.