Objectives: (1) Restore belief in the external world
(2) Revisit the problem of error
(3) Prove that my (i.e., Descartes') essence is thinking
(4) Provide a criterion to tell dreaming from waking
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(1) Restoring Belief in the External World
The imagination uses the pineal gland as a sort of scratch pad.
"I easily understand that the imagination can be actualized in this way, provided a body does exist. And since I can think of no other way of explaining imagination that is equally appropriate, I make a probable conjecture from this that a body exists."
But "this is only a probability." (93)
Can we do better than this?
A more careful summary of the argument:
(3) "this faculty surely cannot be in me, since it clearly presupposes no act of understanding, and these ideas are produced without my cooperation and often even against my will." (97)
(5) But I can't tell which of the possibilities mentioned in #4
holds.
(6) Now, "since God has given me no faculty whatsoever for making this determination, but instead has given me a great inclination to believe that these ideas issue from corporeal things, I fail to see how God could be understood not to be a deceiver, if these ideas were to issue from a source other than corporeal things. And consequently corporeal things exist." (97)
The limitations of the proof of the external world: How well Descartes' argument for the existence of the external world works depends on what sort of property we're dealing with.
But There's a Reason for Optimism:
"There is nothing that this nature teaches me more explicitly than that I have a body that is ill-disposed when I feel pain, that needs food and drink when I suffer hunger or thirst, and the like. Therefore, I should not doubt that there is some truth in this." (97) [Note: 'some truth']
(2) A New Problem of Error
When I have phantom limb pain, I feel pain in limbs that are no longer there.
The Design Flaw: "the nature of the body is such that whenever
any of its parts can be moved by another part some distance away, it can
also be moved in the same manner by any of the parts that lie between them"
(101)
But this doesn't reflect badly on God: "I can think of no better arrangement than that it produces the one sensation that, of all the ones it is able to produce, is most especially and most often conducive to the maintenance of a healthy man. … nothing else would have served so well the maintenance of the body"
(3) Proving Mind and Body are Distinct
There are actually two arguments for this point.
In the Second Meditation, I judged that "nothing else belongs to my
nature or essence except that I am a thinking thing, [therefore] I rightly
conclude that my essence consists entirely in my being a thinking thing."
(96)
(4) Telling Dream from Reality