II. Descartes' Arguments against Materialism
A. Descartes' First Argument: The Argument from Doubt
The main idea behind the argument: Materialists, recall, say that we are identical to our bodies. They say that we just are our bodies. But, recall, Descartes showed us that we can't even be certain that our bodies exist. He also pointed out us that we can be absolutely that certain that we ourselves exist. Thus, since we're certain we exist, but not certain our bodies exist, we can't just be our bodies. We have to be something else. Materialism, however, says that we just are out bodies; so Materialism must be false.
1. The Concept of Numerical Identity
x is identical to y =df. x is the very same thing as y
So this is a relation that absolutely everything bears to itself, and nothing bears to anything else. So it is not the concept of exact similarity. My Sober book is exactly similar to yours, but it is not identical to it.
2. Leibniz's Law
If x has some property that y lacks, then x is not identical to y.
Sober points out how you can use this principle to establish that non-identity of two things. He gives the argument form (262):
1. m has property P
2. b lacks property P
3. If m has property P and b lacks property P, then m is not identical to b.
4. Therefore, m is not identical to b.
Line 3 is an instance of Leibniz's Law.
It is entailed by it.
3. The Argument
Descartes' Argument from Doubt
1. Descartes has an existence than cannot be doubted by Descartes.
2. Descartes body has an existence than can be doubted by Descartes.
3. If (1) and (2), then Descartes is not identical to Descartes' body.
4. Therefore, Descartes is not identical to Descartes' body.
5. If Descartes is not identical to Descartes' body, then MM is false.
6. Therefore, MM is false
P1: No matter what Descartes does, no matter what radical hypotheses Descartes entertains, he is unable to doubt that he exists. Descartes' own existence cannot be doubted by Descartes, for however hard he tries to deceive himself, in order to be deceived, he has to exist to be deceived. So no matter how hard he tries to doubt his own existence, so long as he is doubting, he is existing through it all. Premise 1 is just a convoluted way of saying that Descartes cannot doubt that he himself exists.
P2: Recall the Evil Genius Hypothesis, for example. For all we know, this hypothesis is true. But if it is true, then we have no bodies. Since we can entertain the idea of the Evil Genius Hypothesis being true, we can doubt that our bodies exist. Premise 2 is just a convoluted way of saying that Descartes can doubt that his body exists.
P3: This is just an instance of Leibniz's Law. IF in fact Descartes has a property that Descartes' body lacks, then they cannot be the very same thing. The property that Descartes allegedly has and his body lacks is this: having an existence than cannot be doubted by Descartes. IF in fact Descartes has this property and his body lacks it, Leibniz's Law says that Descartes can't be identical his body.
line 4: [no rationale necessary -- follows logically from 1-3]
P5: Clause (a) of MM says that each person just is identical to his or her body. So, IF in fact Descartes is not identical to his body, then we have a case of a person who is not identical to his body, which contradicts this Clause (a). And if Clause (a) of MM is false, then MM itself is false.
line 6: [no rationale necessary -- follows logically from 4 and 5]
4. A Parody Argument
Well, despite the fact that it is hard to say just how Descartes' argument goes wrong, it is universally regarded nowadays that Descartes argument is unsuccessful. To prove this, we can come up with a "Parody Argument" to show that there is a mistake in Descartes reasoning. The way such parody arguments usually work is that we start with premises that are obviously true, imitate the reasoning in the target argument, and arrive at a conclusion that is obviously false. Then we know that something went wrong with the reasoning in the parody argument (since it took us from true premises to a false conclusion). And since the reasoning is the same in the target argument, we know that something is wrong with that argument as well.
The problem is with the first part of the argument, so we will parody just that part.
The Parody Argument
1. Superman is such that Lois Lane wants
to marry him.
2. Clark Kent is not such that Lois Lane wants to marry him.
3. If (1) and (2), then Superman is not identical to Clark Kent.
4. Therefore, Superman is not identical to Clark Kent.
P1: Lois wants to marry Superman. If Superman flew down and asked her, she'd say Yes. P1 is just a convoluted way of say that Lois wants to marry Superman.
P2: Lois does not want to marry Clark Kent. If Clark Kent cornered her in her office at the Daily Planet and asked Lois to marry him, she's say No Way. P2 is just a convoluted way of saying that Lois does not want to marry Superman.
P3: This is just an instance of Leibniz's Law. IF in fact Superman has a property that Clark Kent lacks, then they cannot be the very same thing. The property that Superman allegedly has and Clark Kent lacks is this: being such that Lois Lane wants to marry him. IF in fact Superman has this property and Clark Kent lacks it, Leibniz's Law says that Superman can't be identical to Clark Kent.
line 4: [follows logically from 1-3]
But something has gone wrong, because we know, independently, that line 4 is false. We have begun with premises that are obviously true and have arrived at a conclusion that is obviously false. We must admit, however, that it is difficult to say just what went wrong.