Some Arguments for Dualism
(and Where They Go Wrong)



Below are two examples of arguments for dualism that are often attributed to Descartes in his Meditations on First Philosophy.

        (A)   
                1. I cannot doubt that my mind exists.
                2. I can doubt that my body exists.
                3. Leibniz’s Law
                4. Therefore, my mind is not identical to my body
 
        (B)   
    1. My mind is  known by introspection.
    2. My body is not known by introspection.
    3. Leibniz’s Law
    4. Therefore, my mind is not indentical to my body.

Notice that each of these arguments uses Leibniz’s Law, which is a principle about identity (which wil discuss extensively in class). Leibniz's Law is a bi-conditional that claims the following:

Leibniz’s Law: Necessarily, for anything, x, and anything, y, x is identical to y if and only if for any property x has, y has, and for any property y has, x has.

Because this is a bi-conditional, it is comprised of two different conditionals statements, (i) and (ii):

(i)    If x is identical to y, then for any property x has, y has and for any property y has, x has.
(ii)    If for any property x has, y has, and for any property y has, x has, then x is identical to y.

Many people claim that argument (A) and (B) above commit the Intensional Fallacy, or the ‘Masked Man’ Fallacy. Intensional Fallacies are committed whenever someone uses the Law of Substitutivity of Identitcals in an Intensional Context.

The Law of Substitutivity of Identicals claims that the following inference is valid:

Fx
x = y
————
Fy

An Intensional Context is one in which the Law of Substitutivity of Identicals fails to hold. For example, if you believe that Superman in strong, you may not believe Clark Kent is strong, even though Clark Kent = Superman. This is because propositional attitudes such as ‘believes that,’ ‘says that,’ ‘thinks that,’ ‘wonders,’ ‘wishes,’ ‘desires,’ ‘doubts,’ etc., are all considered opaque—or non-intensional, or extensional—contexts. Arguments (C), (D), and (E) are all considered invalid arguments; they all commit the Intensional Fallacy.

(C)   
  1. Little Susy knows that 2+2 = 4.
  2. Little Susy doesn’t know that 493827156/123456789 = 4.
  3. Leibniz’s Law
    4. __________________________________
          2+2 is not identical to 493827156/123456789

(D)   
  1. I think my father is a serious guy.
  2. I don’t think that the man in the gorilla suit is a serious guy.
  3. Leibniz’s Law
    4. _______________________
          My father is not identical to the man in the gorilla suit.

(E)   
  1. Prince is widely known to wear really, really tight pants.
  2. Mr. P. Rogers Nelson is not widely known to wear really, really tight pants.
  3.  Leibniz’s Law
    4. ___________________________________
          Prince is not identical to Mr. P. Rogers Nelson


What is going wrong in each of arguments (C)-(E) is that whether an object is known, or thought to be, or wondered about, or wished about, etc., is an irrelevant property (or maybe not even a property at all) as far as Leibniz’s Law is concerned. Leibniz’s Law only says: x = y if and only if any property that an x has, y has. But this is about the properties of x or y; not about what someone else believes (or thinks or knows) about x or y. In other words, propositional attitudes are primarily about the subject who has the attitude, not about the object that plays a role in the content of that attitude. And just because someone may think (or know or believe or not know) something about x, this doesn’t mean that x has the property “being thought by someone to be thus-and-so.”

But perhaps you think that arguments (A) and (B) are not analogous to arguments (C), (D), and (E). After all, arguments (A) and (B) are about what could be known, and what could not; or what is knowable and what is not. Perhaps you think that dualism can be better argued for by argument (F):

(F)
  1. My mind is knowable by introspection.
  2. My body is not knowable by introspection.
  3. Leibniz’s Law
    4. _________________________
         My mind is not identical to my body


Perhaps being knowable—like being doubtable—is a property of an object. And if it is, then one would avoid committing the Intensional Fallacy.

But now consider two people, Rod and Todd. Rod is a Cartesian Dualist and Todd is a Material Monist. If Rod gave argument (F) to support his dualist position, would this convince Todd? Why or why not?

It may help to think about a parallel argument, (G):
 
(G)    
  1. Water is knowable by looking (with the unaided eye).
  2. H2O is not knowable by looking (with the unaided eye).
  3. Leibniz’s Law.
    4. _______________________________
          Water is not identical to H2O


Discussion in class...
Page Last Updated:  Aug. 28, 2009
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