<The following a snippet of Larrys Text, wikified; further development requested>
Dualistic interactionism is the view that mental events and physical events both exist, but neither type of event can be reduced (see reduction) to the other type, and mental events are both causes and effects of physical events. It is also called Cartesian dualism, because it is the kind of dualism that the great 17th-century French philosopher Descartes advocated.
This is one kind of dualism--probably the most popular and well-known, but still only one kind. Two other kinds include parallelism and epiphenomenalism. In addition to the claim that the mental cannot be reduced to the physical and vice-versa, a claim common to all kinds of dualism, dualistic interactionism also includes the claim that there is a causal interaction between mental events and physical events. When Johnny touches a hot stove and burns his skin, he feels a pain. Touching the stove and burning his skin are physical events; feeling pain is a mental event. And when he feels the pain, he says, "Ouch!"--and his saying a word is a physical event. This sort of interaction between the mental and the physical is commonplace, and that it actually happens seems as obvious as anything can be. If we put the belief in that interaction together with dualism, we have got a very common sense theory of mind.
Let's look at some arguments for dualistic interactionism (which, henceforth, we will refer to as "dualism" for short). How would we go about arguing for this theory? I mean, aside from pointing out that it's extremely commonsensical. That's got to count for something; perhaps it counts for a lot indeed. But if we can help it the mere fact that it's a matter of common sense shouldn't be our only reason for accepting a theory. So how would we go about arguing for dualism? We point out the differences between the mental and the physical. So let's do that. Let's go over the differences between mental events and physical events.
First, mental events are not publicly observable. When I touch the hot stove, you may see me whip back my hand and say "Ouch!" but you are not feeling my pain. Unless you're Mr. Spock, or God, you can't as it were get inside my mind and take a look at what's going on in there. And of course it's not just because my mind is hidden beneath my skull. If you knew just where to look in my brain, you wouldn't be able to see thoughts and feelings jiggling around in there. That's just not how it works. So unlike physical events, like fireworks displays, mental events are private, not publicly observable.
Second, mental events are often said not to be spatially located. Where is my pain supposed to be? Maybe you could say in my fingertips, because they hurt. But is that where the feeling is? Does it really make sense to say that the feeling is in my aching fingertips? That sounds a little funny, anyway. A better example would be an emotion like happiness. When I say I'm happy, can I locate my happiness in my head, or does it exist all my body, or something? Doesn't that sound odd? It would seem better to say that my happiness isn't the sort of thing that can be located in a particular place.
Third, more generally, mental events do not seem to have various physical properties which physical events have. For one thing, mental events do not involve anything having mass, or physical motion. We can't weigh a thought. We can't say that a feeling has a velocity of 10 miles an hour. To say such things is to talk nonsense. Now you might say: that's only because mental events are events. You can't say that physical events have mass or velocity either. Point well enough taken; that's true, no event, per se, has mass or velocity. But physical events do involve objects which have mass and velocity. Mental events do not have any components which have mass and velocity. For example, when I think, "I like ice cream," I have a concept of ice cream; and my concept of ice cream has no mass and velocity. Nothing involved in my appreciation for ice cream would appear to have any such physical properties. This is a point to which we will have to return. But on the face of it, this seems pretty obvious.
Fourth, mental events have a certain subjective quality to them, which physical events obviously do not. I mean, for example, what a burned finger feels like, what sky blue looks like, what nice music sounds like, and so on. I'm going to expand on this fourth point at some length. Recently, philosophers have been calling the subjective aspects of mental events qualia, and they also call them raw feels. There is something that it's like to feel pain, to see a familiar shade of blue, and so on; there are qualia involved in these mental events. And the claim is that qualia seem particularly difficult to reduce to anything physical. Just think of what that would involve. You'd be saying: feeling the top of my hand right now, this "raw feel" I'm experiencing right now, is itself nothing more than a physical event.
In fact, there is an article by an American, Thomas Nagel, that came out in the late 70's called "What Is It Like to Be a Bat?" In this article Nagel argued roughly as follows. You are familiar with the fact that bats use a certain kind of sonar, right? They emit high-pitched shrieks which allow them to fly around in dark caves; they can tell how far away the walls are based on how their shrieks echo around in the cave. Bat sonar allows bats to perceive distance, shape, size, and so on, in a way similar to, but obviously different from the way vision works for us. Now Nagel invited us to ask, "So what is it like to be a bat, flying around in the dark using bat sonar?" Surely bats do have experiences; we just don't know what they are. Suppose we were to take apart a bat brain and figure out how the neural apparatus for bat sonar works. No doubt biologists have actually done so, dissected bat brains and so forth. But in understanding how the bat brain works, do those biologists learn what it is like to be a bat? Well of course not. They're mucking around in the grey matter of bats. In order to know what it's like to be a bat, and to have bat sonar, why, you'd have to be a bat. So argued Nagel.
So what's the point of all this? The point is that if indeed there are "bat qualia," then there are peculiar mental events of a sort that only bats have, and we cannot learn what they are like even if we have a detailed understanding of physical events going on in the bat brain. Or to put it even more simply: there is a strange "bat sonar experience" we'll never have. We won't have it even if we learn what physical events in bat brains are associated with bat sonar. So that's excellent evidence that mental events cannot be reduced to physical events; mental events must be regarded as quite a different sort of thing from physical events. Really, you can make the same point without even bringing up weird cases like bat sonar. Just think of your own unique feelings. Do you think that a psychologist could, simply by digging around in your brain and doing tests on your grey matter, ever learn what that feeling was like?
So there are at least four major differences between the mental and the physical, which make it difficult, to say the least, to understand how one might reduce the mental to the physical. Mental events are not publicly observable; they are not spatially located; they do not involve physical properties such as mass and velocity; and there seems to be an irreducibly subjective aspect to them. That seems to give us considerable reason to think that the mind and the body are two totally different categories of being. So there you have one basic, powerful argument for dualism.
But there is another argument that I'm sure weighs heavily in some of your minds. Namely, that you have an immortal souls that can survive your bodily death. Traditional Christianity, like many other religions, teaches that you have a soul which is as different from your body as water is from rock. Your body will die and then your soul will go to heaven, or hell, or who knows where. If you believe this then you practically must believe in dualism. The only way that you can avoid believing in dualism is if you accept phenomenalism, which holds that everything is, ultimately, mental. But in any event you absolutely cannot hold that the soul is reducible to anything physical. If events in your soul were reducible to events in your brain, then when your brain stopped functioning, your soul would cease to exist.
It's not clear to me that this ought to be looked upon as an argument for dualism, simply because those who challenge dualism from a physicalist position will not at all be impressed by it. I mean, suppose you are trying to support dualism against a physicalist, who thinks that everything is, at bottom, physical. Will it be at all impressive to say, "If I have an immortal soul, then mental events cannot be reduced to physical events"? Not at all; the physicalist is precisely the sort of person who denies that you have an immortal soul, and who demands proof that in fact you do have an immortal soul. You may not particularly care that you can't convince the physicalist; but the point is that you should not do something that looks so much like begging the question. Because if you argue: "I have an immortal soul; therefore dualism is true," then it sure does look like you're begging the question. You should prove that dualism is true, and thus that you have an immaterial, non-physical soul, before you try to prove that your soul is immortal!
So as far as arguing for dualistic interactionism goes, let's just stick with the differences between the mental and the physical that I stated before. Now, those are evidence for the dualism part. What about the interactionism part, that is, the claim that mental and physical events causally interact? Here we can just cite observational evidence. We observe that physical events cause all sorts of mental events, and vice-versa. Indeed, it's hard to know how one might argue against the fact that there is such interaction, since it appears to go on all the time. Anyway then, that's all I'm going to say in favor of dualistic interactionism.
This theory has come under strenuous attack from different quarters, especially this century. The basic problem is: If dualism is true, then it is hard to see how mental and physical events can possibly interact causally. How can something totally immaterial affect something totally material? That's the basic problem. But let me break this basic problem into three parts.
First, it is not clear where the interaction would take place. Burning my fingers causes pain, right? Well, apparently there is some chain of events, leading from the burning of skin, to the stimulation of nerve endings, to something happening in the nerves of my body that lead to my brain, to something happening in a particular part of my brain; and then, I feel pain. But the pain is not supposed to be spatially located. So what I want to know is, where does the interaction take place? If you say, "It takes place in the brain," then I will say, "But I thought pains weren't located anywhere." And you, as a dualist, might stick to your guns and say, "That's right, pains aren't located anywhere; but the brain event that immediately leads to the pain is located in the brain." But then we have a very strange causal relation on our hands. The cause is located in a particular place but the effect is not located anywhere. Well, you might say, that might be puzzling but it's not a devastating criticism.
So look at a second problem about the interaction. Namely, how does the interaction take place? Maybe you think, "Well, that's a matter for science -- scientists will eventually discover the connection between mental and physical events." But philosophers have something to say about the matter, because the very idea of a mechanism, which explains the connection between the mental and the physical, would be very strange, at best. Why do I say it would be strange? Compare it to a mechanism that we do understand. Take a very simple causal relation, such as when the cue ball strikes the eight ball and causes it to go into the pocket. Here we can say that the cue ball has a certain amount of force as its mass accelerates across the pool table, and then that force is transferred to the eight ball, which then heads toward the pocket. Now compare that to the situation in the brain, where we want to say a decision causes some neurons to fire and thus cause my body to move across the room. The decision, "I will cross the room now," is a mental event; and as such it does not have physical properties such as force. If it has no force, then how on earth could it cause any neuron to fire? Is it magic? Honestly, how could something without any physical properties have any physical effects at all?
Here you might reply, as some philosophers have indeed replied, as follows. You might say: "Well sure, there is a mystery about how the interaction between mental and physical events can occur; but the fact that there is a mystery doesn't mean that there is no interaction. Because plainly there is an interaction and plainly the interaction is between two totally different sorts of events." Now I expect that some of you may want to say this. But the problem with it is that it does not seem to answer the full power of the objection.
So let me explain the objection more precisely. Let's take as our example my decision to walk across the room. We say: my decision, a mental event, immediately causes a group of neurons in my brain to fire, a physical event, which ultimately results in my walking across the room. The problem is that if we have something totally nonphysical causing a bunch of neurons to fire, then there is no physical event which causes the firing. That means that some physical energy seems to have appeared out of thin air. Do you see? Even if we say that my decision has some sort of mental energy, and that the decision causes the firing, we still haven't explained where the physical energy, for the firing, came from. It just seems to have popped into existence from nowhere.
As our reading says, there is a physical principle, called the "Principle of the Conservation of Energy." According to this principle, "In all physical processes, the total amount of energy in the universe remains constant." Or in a form you may have heard before: in any change anything undergoes, energy is neither created nor destroyed. This is a basic principle you probably learned about in high school physics. So the point is that nerve firings, which are allegedly caused by a totally nonphysical decision, would appear to violate the Principle of the Conservation of Energy.
Now, dualistic interactionists have tried to answer these objections, and other such objections, but most philosophers these days are not impressed by their answers. It has come to the point where, in fact, there aren't very many interactionists around, and there haven't been many for decades. When I say this, I don't mean to imply that dualistic interactionism is false. All I mean to imply is that many philosophers today think it is false, and perhaps also that, if you want to hold onto interactionism yourself, you should try to come up with some effective replies to these objections.