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The following is a portion of Larrys Text; further wikification is encouraged

The theory that the mental can be reduced (see reduction) to the physical is usually called physicalism. It is also called materialism or identity materialism. The term "physicalism" is less misleading because it does not have any misleading connotations; if we called the view "materialism," you might think that we were talking about something that had to do with the desire for wealth, possessions, and so forth, but obviously we're not talking about that. We're talking about a theory about the mind. "Physicalism" is better also because it implies that the mental can be reduced to whatever is physical, meaning, whatever is described ultimately by physics; and as we all know, physics describes a lot more than just matter, it also describes energy. So the view is that we can reduce mental events to events that are made up entirely of matter and energy. And remember what we mean by "reduce" here: "If I reduce X to Y, then whenever I talk about X, I can be understood to be talking about Y instead." So here's the claim: Whenever we talk about mental events, we can be properly understood to be talking about events that are made up entirely of matter and energy.

I suppose that this sounds very strange. I know it sounded very strange the first time I heard it. So let me give you an example, and I'll try to make this as plausible as I can. In fact there is a hackneyed example that is used when philosophers talk about this stuff. For some reason philosophers these days always use pain as an example of a mental event; why not pleasure, I say? There is supposed to be a certain kind of nerve fiber that leads from your limbs to your brain, called C-fibers, and whenever you're in pain, the story goes, your C-fibers are firing; and whenever your C-fibers are firing, then you're in pain. I suppose we might test this claim empirically, with experiments, right? Suppose I sit you down in my lab, and I insert some probe into your spine, right next to your C-fibers; then I find some clever way to cause you pain. You say, "Ouch!" and tell me that you are in pain. I say, "Good!" and sure enough I see that your C-fibers are firing.

My apologies if I'm getting the facts all wrong. But the point is in some such way, we are supposed to be able to study which neural states, and brain states, are associated with which mental states. Now surely that would be very interesting if we were to find that, in order to be in pain, we would have to have firing C-fibers -- wouldn't it? For that matter, a neuroscientist might notice that when you engage in deep thought, the frontal lobe of your brain is extremely active. When we engage in reasoning and complex conceptualization, the neurons in a certain part of the brain are firing; that seems to indicate that there is some sort of close association between hard thought and frontal lobe activity.

So the suggestion now is that the event of feeling pain is just the same as C-fibers firing. That is why Halverson calls the theory "identity materialism"; types of mental events, like pain, are identical to types of physical events, like C-fiber firing. So, pain is reducible to C-fiber firing. This particular theory is also called "the type-type identity theory," since mental event types are matched up with physical event types. And mind you, again, not just matched up or associated with -- but actually the very same as. The pain I experience when I bash my thumb with a hammer is really nothing more than those C-fibers going off.

Now, some of you might hear this and think that I am denying that pain, or anything mental, really exists. In other words, since I'm saying now that mental events are really only physical events, you think that I'm saying there aren't any mental events. But that is not my claim. Mental events do exist; they are simply identical to physical events of a certain kind.

It so happens that there is a theory about the mind, called eliminativism, which says that the mind, mental events, and all the rest of that simply do not exist. The best-known proponent of this theory is an American, Paul Churchland. He says that this talk of mental events, thinking and such, is all just a fiction, just folks tales, which is going to be replaced with more scientific talk, when the science of the brain and the nervous system is sufficiently developed. When that time comes, the eliminativists say, we won't use this out-dated talk of feelings, thoughts, pains, etc. We'll talk about C-fibers firing, and frontal lobes being activated, and so forth. All that talk of love, hate, decisions, beliefs, and so forth, will look antiquated and silly in the same way that talk of ghosts, demons, and witches now looks antiquated and silly. I'm not even going to tell you why some philosophers buy into this theory -- it's pretty unpopular, as you might have guessed -- I'm just telling you that it exists, so I can contrast physicalism with it. Physicalism does hold that mental events exist.

Nonetheless, you still might think that physicalism is a very strange theory, that is totally contrary to common sense. So you might object, as follows. This will be the only objection to physicalism I'll state, but it's rather complex. Just remember now all those differences that we listed between mental and physical events. Mental events are not publicly observable; they are not spatially located; they do not involve physical properties such as mass and velocity; and there is seems to be an irreducibly subjective aspect to them. How on earth could anyone say that mental events are just the same as physical events? There are all these obvious differences, so you can't reduce the mental to the physical! So physicalism has to be wrong.

Here is how the physicalist would reply. Let's go over each of those alleged differences between mind and matter. The first one is: mental events are not publicly observable. The physicalist would say: well, OK, so mental events cannot be seen by other people; but neither can the sorts of physical events we're talking about. A C-fiber firing is not only buried inside your nervous system; it is also not detectable except with special instruments. So we could very well say that C-fiber firing is not publicly observable.

If you're a dualist, you're probably not going to be happy with this reply. You'll probably say that the physicalist is missing the point. The point is really that mental events aren't observable by other people at all. Other people can't jump into my mind and as it were mentally look over my shoulder while I'm feeling pain and having deep thoughts. Maybe Mr. Spock can do that but you and I can't. So, you, or a dualist, might say, what follows from that? What follows is that feeling pain can't be the same as C-fibers firing; other people can probe my nervous system and observe my C-fibers firing but they definitely aren't observing my pain, no matter how carefully they observe my C-fibers or any other part of my nervous system. It doesn't matter; my mental states can't be observed by other people at all! That's how I think the dualist would reply.

How do you suppose the physicalist counter that? Let me play the role of the physicalist. So I would reply by saying that, really, if "publicly observable" just means "observable by other people," then your mental states are publicly observable. Mental events can be observed by other people. So what exactly would I be doing, then, if I did observe your pain? Would I have to feel your pain myself? In other words, in order for me to observe your pain, would I have to have the same aches and hurts that you have? Surely not. Why would you think that? In order for me to observe your pain, it seems to me I simply have to observe the fact that you are in pain. And if, as a physicalist, I hold that pain is the same as C-fibers firing, then all I have to do is observe your C-fibers firing, and voila -- I am observing the fact that you are in pain. The dualist isn't going to be at all happy with this reply; but let's leave that debate and move on to those other supposed differences between mind and body.

The second difference is that mental events are not spatially located. So here's how the dualist will object to physicalism on that point: mental events are not spatially located, but physical events are spatially located. So mental events are obviously different from physical events.

Here's how I, the physicalist, can reply to that point. I can concede that, for example, when I make a decision, I do not see or otherwise perceive my decision as being located anywhere. I am just immediately aware of my decision, once I?ve made it. But does that mean that the event of my decision in fact isn't located anywhere? I mean, simply because I'm not aware of its location, does it follow from that, that my decision doesn't have a location at all? I don't see how that follows. Why can't we just say: a decision is a certain type of event in the brain. When this event occurs, I'm aware of making a decision. But I am not aware of where the decision itself is taking place, namely in my brain. That doesn't mean that the decision isn't taking place in my brain. Why think that?

OK, let's look at the third alleged difference between the mental and the physical. The third difference is: mental events do not involve physical properties such as mass and velocity. The physicalist would say that neural events have electro-chemical properties; but the dualist would reply that the very idea of a mental event being a physical event in the brain and nervous system is absurd, precisely because that would mean that mental events have electro-chemical properties, and other physical properties as well.

I'll bet some of you can predict how our physicalist will reply. As the physicalist, I will say: "Why is that absurd? Let me try to figure out why you think it's absurd that mental events might have physical properties. Maybe you think that since you're not aware of any electro-chemical properties of your mental events, it somehow follows that your mental events don't have any such properties. But once again, that inference just doesn't follow. It assumes that, if all your pains, decisions, and thoughts had some electro-chemical properties, then you'd be aware of those properties. But why think that? Do you have to be aware of all of the properties that your mental events have? Do you have to be aware of everything that is true of the processes of your mind? Of course not. There is a lot of unconscious stuff going on in your mind, a lot of stuff you're not aware of. So then, why not think that your decisions, your pains, and other mental events have electro-chemical properties? The only thing that would make that suggestion absurd is if you thought you had to be aware of everything going on in your mind. But you don't have to be aware of everything going in your mind.

Finally, let's look at the fourth supposed difference between the mental and the physical. It is probably the hardest to deal with: there seems to be an irreducibly subjective aspect to mental events. In other words, there is a first-person, felt, immediate, personal, subjective aspect to mental events. Physical events do not have this subjective aspect; physical events are not qualia. When dualism claims this, it is basically throwing down the gauntlet to physicalism. "Look," the dualist says, "there just is this subjective aspect to my pain; it makes sense to ask, ?What does your pain feel like?? But it doesn't make sense to ask, ?What does your C-fiber firing feel like?? C-fiber firing does not have this subjective aspect. So you can't reduce pain to C-fiber firing."

How could a physicalist reply to this? Well, I think it's not as difficult a problem as some might like to make it out to be. If I'm a physicalist, then my claim is that C-fiber firing, or maybe some other brain event, just is awareness of pain. And so it follows, of course, that C-fiber firing, or the brain event, does have a subjective aspect; and that it does make sense to ask, "What does your C-fiber firing feel like?"

But once again, the dualist is likely to say, "OK, say we open you up and look at your C-fibers firing away. Are we going to see this subjective aspect, the pain qualia that we talked about?" Well, the answer to that is clearly no. But is that a problem? After all, we experience pain only if it's our C-fibers that are firing. If you open my spine up and use scientific instruments to look at my wildly firing C-fibers, then of course you won't be experiencing pain. In order for you to experience pain, your C-fibers would have to be firing. So here's the point then: the dualist wants to object that a physical event cannot have a subjective aspect; the physicalist protests and says there's no good reason not to think that physical events can have subjective aspects. Why couldn't they? We can detect a physical event in someone else without ourselves being in a subjective state. But so what? Why should I expect to be in pain when I see someone else's C-fibers fire? It's only the firing of my own C-fibers that constitutes my own pain. So, President Clinton to the contrary notwithstanding, he cannot feel my pain!

So on all four counts, the physicalist has a way to explain how these supposed differences between the mental and the physical aren't really differences at all. This undermines crucial support for dualism and greatly increases the plausibility of physicalism. It is not so absurd, or at least not so obviously absurd, to say that types of mental events are the same as, and reducible to, some types of physical events. We just have to put these issues in the right light. And physicalism has one very clear advantage over dualistic interactionism, namely, that there is no mystery about how the interaction between mental and physical events takes place. Right? Simply because mental events are, at bottom, themselves types of physical events.

I have given you some ways that the physicalist could reply to the dualist's objections; in so doing, I tried to make it plain how some people can hold what, on first glance at least, looks like an absurd claim, that mental events are simply a type of physical event -- brain or neural events. Now I should note that this is just one kind of physicalism; we may call it neural type physicalism, because it says that mental event types are types of nervous system events. It is more commonly called "the type-type identity theory."

I'm going to present an objection to neural type physicalism. But first I'm going to present another kind of physicalism; I'm going to discuss it very quickly, because mainly I want to use it to make quite clear how neural type physicalism is only one kind of physicalism. In the middle of this century, it was very fashionable to try to reduce mental events to behavior. This view is called analytical behaviorism, because the idea is that we can ultimately analyze, or reduce, talk of mental events and processes in terms of things that humans say, express, and do. In other words, analytical behaviorism says that what a mental event is, is a propensity, a tendency, to display a certain set of behaviors -- words, facial expressions, bodily postures, and actions. If you want to see whether someone believes that God exists, you look at what he says, how he reacts when you say "God exists," whether he goes to church, and so on. And his belief is constituted by those behaviors; in other words, there isn't any more to his belief that God exists than those behaviors. Or to take another example, the good old example of pain. Analytical behaviorism would say that pain is nothing more than the tendency to wince, to grimace, to pull back quickly from the source of something causing bodily damage, to say "ouch" and "that hurts," and similar behaviors. That's all there is to pain!

Well, I'm not going to discuss the merits of analytical behaviorism. Hardly anyone believes it anymore. Good riddance, I say. But this theory is an example of another kind of physicalism. Why? Because it does say that mental events are reducible to physical events; behaviors are physical events, and behaviorism says that mental events are reducible to behaviors. So how does behaviorism different from the sort of physicalism I was talking about last time, neural type physicalism? Well, it's a different type of physical event that mental events are being reduced to. On the one hand, neural type physicalism says that mental events can be reduced to types of neural or brain events. On the other hand, analytical behaviorism says that mental events can be reduced to types of behavioral events. So basically here's a question we might ask: If types of mental event can be reduced to types of physical event, then which types of physical event? Neural type physicalism gives one answer; analytical behaviorism gives a different answer.

This leads me, finally, to that objection to neural type physicalism that I said I was going to give. The objection is this: we have construed the types too narrowly. Maybe there are types of physical events that mental events might be reducible to, that are nothing like neural states. I'd better give you an example if you're to see what I'm talking about. Suppose after some years, superscientists were to build a robot that was given sensory receptors, could talk intelligently on a wide variety of subjects, could carry out a variety of difficult tasks, and even had what appeared to be emotional reactions to its surroundings -- and so forth. In short, somehow, scientists had created what appeared to be a conscious, intelligent machine. Now, there is considerable debate over whether such a machine is possible -- whether it is possible to, as it were, build a mind from scratch. But just assume, for the sake of argument, that such a machine is possible, and that it does have thoughts, perceptions, feelings, and so on. So then mental events are occurring in this machine.

Now let's say our superscientists did not use anything much like the human nervous system to build this robot. They used some sort of special circuitry -- very high-tech microchips and whatnot. In that case, it is not any type of neural or brain event to which we would reduce the robot's mental events. It would be a -- what should we call it? -- a circuitry event! And then in that case mental events could be reducible not just to neural events, but also to circuitry events. So here then is the objection to neural type physicalism: the types we're reducing mental events to is too narrow. To include the mental events of high-tech conscious robots, we should describe the physical events in some way that would include both neural events and circuitry events. Not just neural events.

Or suppose that there is intelligent life elsewhere in the universe. So there might be some alien species that does not have anything quite like our brain or nervous system. Say this species is intelligent, and conscious, and has a mind, but its mental events are identical to a different type of physical event; instead of neurons, they have schmeurons, say. Well then, if we want to be physicalists, then we should allow that mental events can be reduced to neural events, or circuitry events, or to schmeural events! As you can see, the list might go on.

In any case, I think that this neatly refutes neural type physicalism. Neural type physicalists are biased in favor of the physical types that the human species has. So philosophers of mind, trying to be clever, have named this bias species chauvinism. That means that neural type physicalists are irrationally disposed in favor of their own species, when it comes to describing the physical types that mental events reduce to, or are identical to.

Suppose after all this, I still think that dualism is wrong, and I admit that neural type physicalism is wrong; but I still think that some kind of physicalism is still basically right. I think that everything basically has to reduce to the physical; only the physical exists ultimately. So then what are my options? Is there a way for me to hold onto physicalism, so I can get around the species chauvinism accusation? Well, there are basically two ways. The first way is find an even broader type of event, which describes all the different specific physical event types that different species, or robots, might have. This is called functionalism. The second way is to deny that we are reducing mental event types, and talk instead of reducing mental event tokens. This I will call token physicalism. I'm not going to talk about either theory in much depth, because I'll tell the truth -- by now we have gotten quite far away from any issues that were talked about throughout most of the history of philosophy. All of these issues about the different kinds of physicalism have arisen in the last fifty or sixty years or so.

So first I will briefly explain functionalism. Functionalism asks: what do neural events, and circuitry events, and schmeural events all have in common? The answer: it can't be any particular type of physical hardware that they have in common. It's not neurons, microchips, or schmeurons. So what do they have in common? What they have in common is that they are all structurally similar, or functionally similar. What do I mean by that? Well, to similar sorts of inputs, each of these physical types gives a similar sort of output. Let me give a simple example. The human, the robot, and the alien are all going to feel pain when you damage their bodies; and they are all going to have roughly similar reactions, such as announcing that they're in pain, avoiding the source of the damage, and perhaps striking back or getting angry.

So here is a simple definition of "functionalism":

Functionalism is the view that mental events are the same as functional states; and a functional state is a state a physical system is in, when it has a set of sensory and other inputs, together with a set of potential behavioral outputs.

So say I'm thinking about bananas. That mental event is, according to functionalism, a functional state; and that just means a state described by various inputs and outputs. For example, an input would be that someone has said the word "bananas" to me and off I go thinking about bananas; and then a potential output would be to go get a banana and eat it. So my mental event, thinking about bananas, is just a functional state, and the functional state is specified by the inputs of the event and the outputs of the event.

Well of course there is a lot we could say about functionalism. For one thing, if all we have at our disposal, to describe mental events, is their inputs and outputs, then have we really described the mental event itself adequately? It would be a little like saying that an oak tree could be adequately described like this: "Oak trees result from acorns (that's the oak tree input) and they are cut up to make tables (that's the oak tree output)." Yeah, but (you might ask) what is an oak tree itself? You might ask a similar thing of functionalists. "OK, so mental events have certain inputs and outputs; I can accept that; but what is a mental event itself?" Unfortunately, we don't have time to go into it. But for what it is worth, functionalism is probably the most popular theory of mind today. So obviously a lot more can be said about it.

I said there is a second way to get around the species chauvinism objection. This can be called token physicalism. More commonly called "the token-token identity theory." Whatever you want to call it, I'll define as follows:

Token physicalism is the view that tokens of mental events may be reduced to tokens of physical events.

The idea here is that we are giving up trying to give general accounts of mental event types. So for example we won't try to reduce the whole category of pleasure, or the whole category of pain, to any single mental event type. We'll focus in on individual, single, pleasures and pains. And we say, of those mental event tokens, that each one is identical to, and reducible to, some physical event token. In our case, we might say that an individual pain is the same as an individual instance of my C-fibers firing. But we might say something quite different about the pain of an alien from Alpha Centauri. The point in either case, though, is that it's tokens of mental events that are reduced to tokens of physical events.

I'm just going to give one little objection to this theory. Namely, what does the following phrase mean? -- "Token of a mental event." Token physicalism can't tell us. I mean, suppose a scientist had reduced a slew of mental event tokens to physical event tokens. So I'd say, "Great work! But just what do all of those mental event tokens have in common, that makes us say that they are tokens of mental events, as opposed to any other kind of event?" What distinguishes the mental event tokens from tokens of other kinds of events? That's the question: What distinguishes mental event tokens? Dualism, remember, says that the mental is an ultimate, fundamental category of being; it can't be explained in terms of anything nonmental. So dualism doesn't have to answer this question. And neural type physicalism at least promises to answer the question; it says that we will discover just exactly what all the different mental events have in common, which makes them all mental events; and it's going to be some special type of event in the brain.

But token physicalism can't answer this question; it can't tell us what mental event tokens have in common; and why not? Because if any theory tells us what mental event tokens have in common, then the theory is describing mental event types. Just think: that is precisely what mental event tokens have in common: they are all tokens of mental event types. What do human pain and alien pain in common? They are both tokens of the type, pain. So, if we describe what all mental event tokens have in common, then we have for that very reason described a mental event type! And then the token physicalist would have to talk about mental event types; and that means we?ve basically given up token physicalism. Well, that's the only thing I'm going to say about token physicalism. My usual disclaimers apply.

In the interests of mercy, let me say that I'm not trying to make you believe that we can't know which theory of mind is correct -- I don't want you to leave class being skeptics about philosophical questions about the mind. Generally, in the interests of objectivity and general intellectual responsibility, I feel it is my duty to tell you about most of the leading philosophical theories of the different subjects we're studying, and some arguments for and against them. So, the mere fact that I'm not telling you that one theory is The Truth doesn't mean that I have no views about what The Truth is. In fact, I do think that one particular theory of mind is better than the others -- I'm just not telling what it is! And the mere fact that I'm presenting objections to all of the theories I?ve stated doesn't mean that I think we can't know which one is correct, or that they are all false; all I'm trying to do is to introduce you to the issues. Philosophical issues are more complicated than you might have thought when you came into this class. So if you want to have really well-informed views on these issues, it's just the same as having really well-informed views on issues in chemistry, or in political science, or in computer programming. It's going to take research and long hard thinking.