1. Even if every mental state is a brain state, it is quite clear that not every brain state is a mental state: not everything going on in the brain manifests mentality. So what distinguishes the brain states that are mental states from the brain states that are not? This question cannot be evaded.
The distinguishing feature cannot be anything intrinsic to brain states qua brain states. To put it another way, the biological, electrochemical, and other terms appropriate to the description of brain phenomena are of no help in specifying what makes a brain state mental. Talk of axons, dendrites, synapses, diffusion of sodium ions across synapses, etc. is not the sort of talk that makes intelligible why a particular complex state of Jones' brain is his intense elation at getting his neuroscience text accepted for publication.
2. To help you understand what I have just said, I offer an analogy. Even though every valve-lifter is an engine part, it is quite clear that not every engine part is a valve-lifter. So what distinguishes the engine parts that are valve-lifters from the parts that are not?
The distinguishing feature cannot be anything intrinsic to engine parts qua physical objects. The mechanical, chemical, electrical, metallurgical and other terms appropriate to the description of engine parts are of no help in specifying what makes an engine part a valve-lifter. A metallurgist might tell us everything there is to know about the physical properties of those engine parts that are valve-lifters. But knowing all of that, I do not yet know what makes the part in question a valve-lifter. Similarly, I don't know what makes a certain heavy object under my hood a battery just in virtue of knowing all the electrochemistry involved in its operation.
3. The obvious thing to say at this point is that what make an engine part a valve-lifter or a battery or a generator or a transmission is its function. Physical composition is irrelevant. What makes a part a valve-lifter is the causal role it instantiates within the 'economy' of the engine. A thing is a valve-lifter in virtue of the job it does when properly connected to valves, cams, etc. Its being a valve-lifter is not intrinsic to it. Its being is its function within a system whose parts are causally interrelated.
I stress that physical composition is irrelevant. Anything that does the job of a valve-lifter is a valve-lifter. Anything that does the job of a modem is a modem. There is more than one implementation of the modulation-demodulation function. The function is 'multiply realizable' as we say in the trade. Of course, not every physical substratum supports the function: not even in Eskimo land could valve-lifters in internal combustion engines be made of ice.
Another important point is that a particular thing that functions as a valve-lifter can assume other functions, that of paper-weight for example. So not only are causal roles typically multiply realizable, causal role occupants or realizers are typically multi-functional.
I think we are all functionalists when it comes to things like valve-lifters, screwdrivers, switches, and modems. Anything that modulates/demodulates is a modem regardless of the stuff inside the
box that realizes or implements the function. For all we care, there is a colony of leprechauns inside the box that chop up the analog input into digital bits. If it does the job of a modem, it IS a modem.
Can we apply this functionalist model to the mind?
4. If there is nothing intrinsic to brain states that explains why some of them are mental states, then the naturalist must look to the extrinsic or relational features of brain states. How do they function? What causal role do they play? How do they stand in relation to inputs and outputs? How did they come into being? What are they good for?
One answer is the functionalist theory that causal role is what makes a brain state a mental state. What makes a mental state mental is just the causal role it plays in mediating between sensory inputs, behavioral outputs and other internal states of the subject whose state it is. The idea is not the banality that mental events have causes and effects, but that it is causal role occupancy, nothing more and nothing less, that constitutes the mentality of a mental state. The intrinsic nature of what plays the role is relevant only to its fitness for instantiating mental causal roles but not at all relevant to its being a mental state.
That's the basic idea. What makes a brain state a desire is the causal role that state plays. There is nothing intrinsic to the brain state itself that could tell you that it was a desire for a beer rather than
an intention to paint the bathroom, or a memory of a trip to the Grand Canyon. In their intrinsic nature mental states are just brain states; it is only their external relations that confer upon them mentality.
5. Here is one problem. It seems clear that my intention to clear brush could not have been a desire for a cold beer. Nor could it be an intention to paint the bathroom. The act of intending is individuated by its intentional content (to clear brush; to pain the bathroom): the content enters into the description of the act. This entails that the act could not have been an act having a different content.
But if it is causal role occupancy that makes brain state B an intention to clear brush, then B could have been an intention to paint the bathroom, had its causal relations been different. Since this is absurd, it cannot be causal role occupancy that makes B an intention to clear brush. The fact of intentionality refutes functionalism.
Compare the valve-lifter. A particular engine part is a valve-lifter in virtue of the causal role it plays in the engine. But that part might not have been in the engine; it might have been on my desk weighing down papers, in which case it would have been playing a different causal role. There is no problem in this case because valve-lifters lack content, or directedness to an object. A valve-lifter is not about anything. But an intention is. And this aboutness is intrinsic to it, which is why it cannot be captured extrinsically in terms of functional role.
So one should not suppose that qualia are the only problems for functionalism. Intentionality is just as much of a problem. Compare the Martian neuroscientist argument given earlier.
Besides, one is superficial and thoughtless if one imagines that a clean separation can be made between qualia and intentional phenomena. But that's a separate post.
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