PRINCIPLES
OF PHILOSOPHY
Mind Body Dualism
While "I think,
therefore I am" is Descartes' most famous one-liner, substance dualism is
his most famous philosophical position. According to substance dualism, our
minds and our bodies are two distinct substances capable of existing apart.
The argument for substance
dualism relies heavily on all the work that Descartes has done up until this
point. The argument rests on the claim that the essence of mind is thought
while the essence of body is extension, and this duality of essences implies a
duality of corresponding substances. The full argument can be broken down into
seven steps: (1) If I can clearly and distinctly perceive something, then God
makes something that exists that corresponding to my clear and distinct
perception, otherwise God would be a deceiver. (2) If I can clearly and
distinctly perceive X and Y as complete things whose principal attributes
exclude each other, then God can make X and Y exist apart from each other. (3)
If X and Y can exist apart from each other (whether or not they really do) then
they are really distinct. (4) I can clearly and distinctly perceive mind as a
complete thing to which extension does not pertain. (5) I can clearly and
distinctly perceive body as a complete thing to which thought does not pertain.
(6) Therefore, God could bring it about that mind and body exist apart from
each other. (7) Therefore, mind and body are really distinct.
Analysis
What is Descartes'
motivation in arguing for substance dualism? One practical upshot of this claim
is the possibility of life after death. If mind and body can exist apart, then
our soul can survive the death of our body. More important than that, though,
(for Descartes, at least) is that substance dualism ensures that physics boils
down to geometry. Physics is the study of physical substances. If things like
sensations and thoughts belonged to physical substance then physics would have
to account for them. This would be a problem for Descartes, because sensations
and thoughts cannot possibly be expressed mathematically, or mechanistically
for that matter. They would destroy the entire neat, mathematical, mechanistic
expression of the physical world. Descartes, therefore, takes these items out
of the realm of the physical (and, thus, of physics) by claiming that in
addition to physical substance there is an entirely different substance in the
world: mental substance. Substance dualism, then, is a necessary precursor to
the rest of the text.
Since this claim is so
significant for Descartes' project, it is important to get clear on every step
of the argument. There is a lot going on in every premise, and so a lot of
unpacking must be done in order to get a perfect grasp of how the whole thing
is supposed to work.
Step (1) seems clear
enough. This is just a corollary of Descartes' guarantee of clear and distinct
perception. Step (2), though, confronts us with a question: why is it important
that the principal attributes exclude each other? Why is step (2) not just,
"If I can clearly conceive of X and Y as complete things then God could
bring it about that they exist apart"? To understand why this is necessary
consider the following example of bad reasoning: I know that Lilly is a woman.
I, therefore, know that Lilly has no talent for football. This reasoning fails
because there is nothing in the idea of being a girl that excludes the
possibility of being good at football. One can be both a girl and talented at
football. Imagine this other example: I can conceive of a square while only thinking
of four equal sides. Therefore, I can conceive of a square without conceiving
of shape. It is true that I can think of a square by thinking only of four
equal sides. However, having four equal sides is just a way of being shaped.
When you think about something having four equal sides, you are necessarily
also thinking about shape.
Now consider what would
happen if Descartes' second step lacked the proviso about exclusion. The
argument would go like this: I know that thought is the essence of mind. I know
that extension is the essence of body. I, therefore, know that mind and body
have different essences. Can he draw this conclusion from the premises? In
order to draw this conclusion, he has to show that extension and thought are
not like femininity and football talent, or squareness and shape, but more like
squareness and circularity. In other words, he must show that they exclude each
other. What if thought was just another way of being extended (that is, a mode
of extension), or extension was a kind of thought? Then the case would be
exactly analogous to the square/shape example. By clearly and distinctly
conceiving of mind as a thinking thing, you would also secretly be conceiving
of body and vice versa. In other words, you would not be able to clearly and
distinctly conceive of them apart at all. And if you cannot clearly and
distinctly conceive of them apart, then the whole game is lost.
The next obvious question
that arises is how Descartes actually proves that extension and thought exclude
each other. Nowhere do we see him explicitly proving this. In fact, steps (4)
and (5) are terribly compressed. Both contain two claims: P is the essence of
S; P is not connected to Q.
First, how does Descartes
know that thought is the essence of mind and extension is the essence of body?
For this part of the claim he is resting on the suppressed premise we saw in
the last section: P is the essence of S if and only if I can conceive of S
attributing only P to it. He sees that he can conceive of mind by attributing
only thought to it and of body by attributing only extension.
Now, though, the big
question: How does he know that extension and thought are not like squareness
and shape? How does he know that they exclude each other? Though he never
explicitly states his reasoning, it is not that difficult to figure out.
Extension and all of its modes can be captured by mathematical language. It
seems intuitive to believe, however, that the modes of thought (experience or
consciousness) cannot be expressed mathematically. After all, one would be hard
pressed to imagine how to mathematically express the experience of seeing red.
Since it is of the nature of extension to be expressible in mathematical terms,
and of the nature of thought to lack this possibility, he is able to conclude
that neither is a mode of the other. Extension and thought exclude one another.
Given all this, we can now
see clearly how Descartes is able to conclude the argument. He knows that mind
and body have different essences that are mutually exclusive. That means that
when he clearly and distinctly conceives of one, he is not secretly also
conceiving of the other. He also knows that God can bring about the separation
of anything we can clearly and distinctly perceive. Since we can clearly and
distinctly perceive of mind without body and of body without mind, God can
bring it about that mind can exist without body and body without mind. In other
words, they are really distinct.
REFERENCE
Descartes, Rene. Principles of
Philosophy. Trans. Valentine Rodger Miller and Reese P. Miller.
Boston: Kluwer Academic Publishers, 1991.
Descartes, Rene. The Philosophical
Writings of Descartes. Trans. John Cottingham, Robert Stoothoff,
Dugold Murdoch. New York: Cambridge University Press, 1985.
Cottingham, John, ed. The Cambridge
Companion to Descartes. New York: Cambridge University Press, 1992.
Des Chene, Dennis. Spitis and Clocks:
Machine and Organism in Descartes New York: Cornell University Press,
2001.
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