1. Views on the Mind-Body Problem
substance dualism: mental states and processes are states and processes of a nonphysical substance.
substance physicalism: there are no nonphysical substances. Mental states and processes are states and processes of a physical entity (more specifically, a biological organism), not of a nonphysical substance.
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.]
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|
The two arguments Sober discusses in chapter 19 are both like the argument above that Charlotte is not identical with Charlie. The first involves the (supposed) property of indubitable existence, which Descartes thinks the mind has but the body does not have. The second argument involves the property of divisibility, which Descartes thinks the body has but the mind does not have. I'll just focus on the first.
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. 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.
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.
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!