1. Views on the Mind-Body Problem
substance dualism: mental states and processes are states and processes of a nonphysical substance.
minimal physicalism: mental properties supervene on physical properties. (See Kim, p. 13.)
property physicalism: mental states and processes are physical states and processes. That is, not only are they states and processes of a physical entity, they will turn out to be analyzable and explainable in terms of physical states and processes. (Sometimes this is put by saying that mental states and processes are reducible to physical states and processes, but this terminology is potentially misleading.)
property dualism: although mental states and processes are states and processes of a physical entity, they are not physical states or processes. [Note: it is possible to be a property dualist about some mental states and processes but not others. For example, some hold that conscious experience is not a physical state, although other mental states and processes are.]
An important and difficult question here is: what does it mean for a state or process to "be physical"? It can't mean: "be one of the states or processes described by physics," because physicalists want biological, geological, etc. properties to count as physical. It can't mean: "be a state or process of a physical object," because that would rule out property dualism by definition.
Different ways of conceiving of what's physical lead to different understandings of the term "property dualism," and this can be very confusing. The most prominent contemporary philosopher who identifies himself as a property dualist is David Chalmers, but he is not a property dualist in Kim's sense, since Kim describes property dualism as a form of physicalism, and Chalmers is not a physicalist!
Kim defines "being physical" in terms of reducibility: a state or process is physical in the relevant sense if it can be completely analyzed and explained in terms of the properties, states, and processes described by physics. This seems to leave open the possibility that a property could supervene on physical properties without being definable in terms of them, so that one could be both a physicalist and a property dualist. And in fact Kim treats property dualism as a type of physicalism.
On the other hand, Chalmers defines physical properties in terms of supervenience: a property is "physical" if and only if it supervenes on the properties described by physics. For Chalmers, therefore, property dualism is not a form of physicalism.
interactionist dualism: mental processes cause physical processes, and vice versa
epiphenomenalist dualism: physical processes cause mental processes, but mental processes do not cause physical processes
2. The Principle of the Indiscernibility of Identicals
Indiscernibility of identicals: if x = y, then x has all the properties y has (and vice versa)
The identity symbol '=' here means numerical identity, i.e. being the very same thing (not just being exactly similar). Two pieces of chalk from the same box, or two cars off the same assembly line, might be exactly similar to one another, but they cannot be numerically identical to one another. Every object in the universe is numerically identical to only one thing, namely itself. So the idea of the Indiscernibility of Identicals is that if x is the same thing as y, then x must have all the properties y has.
This may seem trivial, and in a sense it is. Of course everything has exactly the properties it has! For instance, Curtis Brown = Curtis Brown. Therefore, if the principle is correct, then Curtis Brown has all the properties that Curtis Brown has.
When applications of the principle become a little more interesting is when we have different ways of referring to the same thing.
Example: I come into class one day, pick up a nice new piece of chalk, and name it "Charlie." Half an hour later, I pick up a piece of chalk and name it "Charlotte." I may not be sure whether the piece of chalk I named "Charlotte" is the same as the piece of chalk I named "Charlie."
Therefore, I'm not sure whether Charlotte = Charlie.
However, I do know (by the indiscernibility of identicals) that if Charlotte = Charlie, then Charlotte has all the properties Charlie does, and vice versa. Suppose I inscribed Charlie's name on it when I named it. If Charlotte = Charlie, then Charlotte has the name "Charlie" inscribed on it. So if I check and find that Charlotte does not have "Charlie" inscribed, I can conclude that Charlotte is not identical with Charlie.
Most arguments for mind-body dualism are like this: they attempt to find a property that the mind has but the body does not (or vice versa), and conclude that they are not the same thing.
An Argument for Dualism from the Indiscernibility of Identicals
There are some slightly odd things about chapter 2 of Kim, Philosophy of Mind, second edition. One is that, after suggesting that we should talk about "mentality" rather than about "minds" to allow for the possibility that substance dualism is incorrect, he then presents several arguments which revert to talking about the relation between minds and bodies instead of the relation between mental and physical processes. I suppose the reason is that he is discussing substance dualism. But I think it's better to express the arguments in terms of properties, because the conclusions of Kim's arguments (minds are not identical with bodies, I am not identical with my body, etc.) don't actually imply dualism; since physicalists don't think "the mind" is a thing at all, they also don't think that it is the same thing as the body!
Another odd thing is that he presents seven arguments for substance dualism, but doesn't devote substantial critical discussion to any of them. I will focus on an argument very similar to Kim's Argument 1, except that I've modified it to concern mental processes rather than "minds."
Lots of similar arguments have been offered. One problem with many of them is that they simply beg the question. For example, according to Descartes the mind has the property of thinking, but the body does not. (Compare Kim's Argument 4.) But the claim that the body doesn't think simply begs the question by assuming physicalism is false. Physicalists, of course, maintain that the body does think!
1. The indubitable existence argument.
At first glance, the "indubitable existence" argument looks better than this, though. It seems that we have independent reasons to think that the mind exists indubitably but the body doesn't, and that we can use this fact to prove that they're different.
Definition of indubitable existence: x has the property of indubitable existence if and only if I cannot doubt that x exists.
Descartes tries to prove two things in Meditation 2.
1. My thoughts have the property of indubitable existence.
2. No physical process has the property of indubitable existence.
1 is supported by the observation that even if I were being deceived by an evil genius, I still could not be mistaken in thinking that I have the thoughts I do. 2 is supported by the theoretical possibility that an evil genius could deceive me into believing that I had a body even though I was in fact a disembodied spirit.
But if 1 and 2 are correct, and the principle of the indiscernibility of identicals is also correct, then it follows that my thoughts cannot be identical with any physical process. That is, we have the following argument:
1. My thoughts have the property of indubitable existence.
2. No physical process has the property of indubitable existence.
3. If x and y have different properties, then x is not identical with y
4. My thoughts are not identical with any physical process
2. Response to the indubitable existence argument.
The general form of the argument cannot be valid, because there are other arguments with the same form that are clearly invalid. For example:
Superman has the property of indubitable identity with Superman.
Clark Kent does not have the property of indubitable identity with Superman.
Clark Kent is not identical with Superman.
The argument above looks to have the same form as the indubitable existence argument. Therefore either they are both valid, or they are both invalid. But the argument about Superman seems to be invalid (since the premises are true but the conclusion is false). So the argument from indubitable existence must be invalid also.
What exactly is wrong with the argument, though? One way to put what the problem seems to be is this. Indubitable existence (and indubitable identity with Superman) are not properties of a thing considered by itself. They are properties of a thing when it is thought of in a particular way.
Here's an analogy. I have the property of being the oldest person in Chapman 18 when I'm in the room with students. However, I do not have the property of being the oldest person in Chapman 18 when I'm in the room with the rest of the Philosophy faculty members. So "the property of being the oldest person in Chapman 18" is not really a property because it is incomplete. To completely specify a property we'd have to say something like "the oldest person in Chapman 18 at 12:30 on October 31, 2007."
Similarly, the description "indubitable identity with Superman" is incomplete. We need to add how Superman is being thought of. Superman has the property of "indubitable identity with Superman when thought of as 'Superman'," but Superman does not have the property of "indubitable identity with Superman when thought of as 'Clark Kent'."
Finally, returning to the relation between thoughts and physical processes, we have the same problem. "Indubitable existence" is not a completely specified property. "Indubitable existence when thought of as a thought" might be a perfectly good property, and my thoughts all clearly have it. But this doesn't prove that thoughts are not physical processes. If thoughts are in fact physical processes, then some physical processes also have the property of indubitable existence when thought of as a thought!
Mental Causation as a Problem for Dualism, Take I
Any argument for physicalism is automatically an argument against dualism, and we'll be seeing some of these soon. But if we focus on the negative, the main argument against interactionist dualism, ever since Descartes' own time, has simply been that there is no way that a nonphysical substance (or property or process or event) could cause a change in a physical substance (i.e. cause a physical property or process or event).
There are lots of ways to try to formulate this general argument. Here are a couple:
1. The physical universe is a closed system: every physical event has a physical explanation.
2. If nonphysical minds could cause physical bodies to do something, then there would be physical events with no physical explanation.
3. Nonphysical minds cannot cause physical bodies to do anything.
1. Conservation of mass-energy: the amount of mass-energy in the universe is constant.
2. If the nonphysical mind could cause physical events that would not have occurred otherwise, then the amount of mass-energy in the universe would have been increased.
3. A nonphysical mind cannot cause a physical event that would not have occurred otherwise.
Suppose we are convinced by arguments such as these. Notice that there are two very different ways of abandoning interactionist dualism. (1) One can give up the interactionism but hold onto the dualism. That gives us an epiphenomenalist position. (Or possibly occasionalism or pre-established harmony!) (2) Or one can hold on to the interactionism, but give up on the dualism. That gives us one or another version of physicalism.
Mental Causation as a Problem for Dualism, Take II: The Pairing Problem
Kim offers a different and original version of an argument against dualism involving mental causation. In a nutshell:
Consider objects a, b, and c. a and b are exact duplicates which share all their intrinsic properties. For example, suppose that a and b are identical loaded handguns, and c is a person. Now let A and B be the event of a and b firing, respectively. Suppose that these firings are also alike in all intrinsic respects: same type of ammunition, the bullet leaves the barrel at the same velocity, etc. Finally, let C be the event of c's death.
It can happen that A causes C while B, despite being intrinsically exactly like A, does not cause C.
[I'm using capital letters for events and lower-case letters for objects, here. Intrinsic duplicates are things that have exactly the same intrinsic properties. An intrinsic property of something is a property that depends only on the thing itself, not on its relations to other things. So mass is an intrinsic property, but distance from the sun is not.]
What explains why A caused C even though B, which is exactly like A in all intrinsic respects, did not cause C?
A bad answer: there is a causal chain from A to C, but there is no causal chain from B to C. This is a bad answer because a causal chain is just a sequence of cause-effect relationships, and each event in the sequence raises the same problem all over again.
Kim's suggestion: the only answer that seems plausible has to do with spatial relations between A, B, and C. In particular, c was in front of gun a but not in front of gun b.
Kim proposes this as a general answer to the question of how intrinsically identical events can have different causal effects. The idea is that in general, whenever an event A has a causal effect on something that an intrinsically identical event B does not have an effect on, the reason must be that A is spatially related to that thing in a way that B is not.
If we accept this as a general answer, then we have a problem with dualist mind-body causation. How can mind a have a causal effect on a body that intrinsically identical mind b has no effect on? Kim says: (a) the only possible answer is that a is spatially related to the body in a way that c isn't, but (b) this answer won't work in this case because for the dualist, minds do not stand in spatial relations.
[I'm not sure how persuasive I find this. I'm not sure it actually seems right to me that spatial relations are the only possible explanation for differential causal effects. Far-fetched example: suppose that magic actually works. I can take my magic wand and make a dove appear anywhere I wish. I decide I want a dove to appear atop the Eiffel tower. I twitch my wand and poof! The dove appears.
Now, why did the dove appear atop the Eiffel tower instead of somewhere else? Because that's where I intended for it to appear, not because of any spatial relation between me and the tower. A world in which this worked would be weird, but it doesn't seem incoherent.
But that's exactly how the dualist thinks mind-body interaction works. My mind wills my arm to go up, and it does. Why did my arm go up rather than yours? Because this arm is the one my mind intended to go up. I don't think that the world actually does work this way, but I don't see that Kim's argument shows that it couldn't.]
There's something weird about the philosophy of mind (and philosophy in general). The arguments about dualism and physicalism that philosophers like to discuss tend to be deductive and a priori. They are arguments that dualism or physicalism must be true, or alternatively that it cannot be true.
However, I'm inclined to doubt that any such arguments will be successful. In the end, it seems to me that physicalism and dualism are alternative hypotheses about how the world works, and like all hypotheses, they need to be evaluated in terms of their plausibility in the face of the evidence and in view of considerations such as elegance and simplicity. The evidence may not force us to accept one or the other, but it can make one position more probable.
I think the right questions to ask are questions like the following. We know that there are detailed, complex, and widespread connections between what goes on in the brain and what goes on in the mind. What is the best explanation of these connections? Physicalism says that the best explanation is that the mental processes just are physical processes in the brain; this explains how they can cause and be caused by physical phenomena and why the connections are so extensive. The rival explanation, dualism, says that mental processes happen only in a nonphysical substance, not in the brain, but that there are many, many causal connections between what happens in the brain and what happens in the mind.
My own view is that physicalism gives a simpler, more powerful explanation of the phenomena and that therefore it ought to be preferred. (In fact I would go further: I think that dualism in its current and historical forms has so little detail that it can't really explain anything at all. No one purports to know anything whatsoever about the nature of nonphysical substances, how they interact causally with other nonphysical substances and with physical ones, etc. etc. Dualism isn't really well-enough developed to be a theory at all; it amounts to little more than the assertion that they physical world isn't enough.) But whether I'm right or wrong about which way the evidence points, I strongly suspect that this will turn out to be the most fruitful way to look at the issue. (I would say the same thing about other issues that have traditionally been regarded as philosophical, such as the existence of God.)