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An Argument for Skepticism
P1. If S knows that P, then it isn't possible that S is mistaken in believing that P.
P2. It is possible that s is mistaken in believing that P.
C. Therefore S doesn't know that P.
If I have a reason to doubt P, then I do not know that P.
Foundationalism: to reject everything that you have ever known and start from the beginning.
Demolish the foundations of the house that you built and build it again from only things you are
certain of. Anything that we have a reason to doubt is to be discarded. This process is called
Radical skepticism: neither my beliefs about the external world nor my belief that 2+2=4 are
Reasons to doubt
1The senses deceive us about small and distant objects (illusions)
2I may be a madman who thinks I have true perceptions.
3I may be dreaming
4God may have made me in a way that I am systematically deceived about what goes on around
me. We're told to be created by a good God, but how do we really know he's good?
The Evil Demon Hypothesis
Possible World 1: God gives me true beliefs. God is not a deceiver.
Possible World 2: God gives me false beliefs. God is a deceiver. God is an Evil Demon.
Can I be certain that I am living in possible world 1 rather than 2?
Are there any beliefs that is indubitable?
Possible indubitable belief: There is a god who causes me to have the beliefs that I have.
Response: But maybe I'm the source of those beliefs.
Counter-response: If I am the source of my thoughts, then I must exist, right?
Objection to counter-response: But I have convinced myself that there is absolutely nothing in the
world, including no minds. Does it follow that I too do not exist?
Response: No, if I convinced myself of something then I certainly existed.
Conclusion: Cogito Ergo Sum. I think, therefore I exist.
? What is I? I am not a body, but a thinking thing.
Other indubitable beliefs:
1I feel pain.
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