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• http//pantheon.yale.edu/kd47/e-page.htm
• http//www.jimpryor.net/teaching/index.html
• You must enroll in a discussion section.
• Borderline grades will be decided based on
  section participation.

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Lecture 1

• Skepticism

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• Some background logic

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The Logic of Skepticism

• Some basic logic Modus Ponens
• (1) If we have eggs, then Jim went to the store.
• (2) We have eggs.
• (3) ? Jim went to the store.

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The Logic of Skepticism

• Some basic logic Modus Ponens
• (1) If we have eggs, then Jim went to the store.
• (2) We have eggs.
• (3) ? Jim went to the store.
• - Logic deals with the form, not the content of
  arguments. So, to consider only the form of this
  argument, we will cut the content out.

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The Logic of Skepticism

• Some basic logic Modus Ponens
• (A)
• (1) If we have eggs, then Jim went to the store.
• (2) We have eggs. (A)
• (3) ? Jim went to the store.

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The Logic of Skepticism

• Some basic logic Modus Ponens
• (1) If we (A)veeggs, then Jim went to the store.
• (2) (A)
• (3) ? Jim went to the store.

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The Logic of Skepticism

• Some basic logic Modus Ponens
• (B)
• (1) If we (A)veeggs, then Jim went to the store.
• (2) (A)
• (3) ? Jim went to the store. (B)

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The Logic of Skepticism

• Some basic logic Modus Ponens
• (1) If we (A)veeggs, then Jim (B)t to the store
• (2) (A)
• (3) ? (B)

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The Logic of Skepticism

• Some basic logic Modus Ponens
• (1) If we have eggs, then Jim went to the store.
• (2) We have eggs.
• (3) ? Jim went to the store.

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The Logic of Skepticism

• Some basic logic Modus Tollens
• (1) If we have eggs, then Jim went to the store.
• (2) Jim didnt go to the store.
• (3) ? We dont have eggs.

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The Logic of Skepticism

• Some basic logic Modus Tollens
• (1) If we (A)veeggs, then Jim (B)t to the store
• (2) Not (B)
• (3) ? Not (A)

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The Logic of Skepticism

• Some basic logic Modus Tollens
• (1) If A, then Bt to the store
• (2) Not B
• (3) ? Not A

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The Logic of Skepticism

• Some basic logic Modus Tollens
• (1) If A, then Bt to the store
• (2) Not B
• (3) ? Not A
• Skepticism in contemporary philosophy is usually
  put forth as a Modus Tollens argument.

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René Descartes 31 March 1596 11 February
1650 Father of modern philosophy (A warning
from history to the dangers of early rising.)
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• Descartes Argument for Skepticism

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• Descartes was concerned with securing solid
  foundations for his knowledge.
• So he doubted everything and searched for
  something he knew.
• He famously found that he knew he was thinking
  Cogito ergo sum.
• But we are concerned with his argument for
  doubting the existence of the external world.

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• Descartes is sitting by the fire looking at a
  piece of paper.
• He seems to know that he is looking at a piece of
  paper.
• But then he remembers that his senses sometimes
  deceive him. Even worse
• Sometimes, when dreaming, he thinks hes awake.
  So he might now be dreaming and so not really
  perceiving the paper.

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Have you ever had a dream, Neo, that you were
so sure was real? What if you were unable to
wake from that dream? How would you know the
difference between the dream world and the real
world? - Morpheus
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Good Case Bad case
There is no perceptual difference between being
in the real world and being in the Matrix / In a
dream. 2. So you dont know youre not dreaming.
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Skepticism

• In contemporary terminology, the ground for
  doubt proposed by Descartes can be put like this
  
• The Cartesian-style argument for Skepticism can
  now be put like this
• (1) If I know that Im looking at a piece of
  paper, then I know Im not dreaming
• (2) I dont know that Im not dreaming
• (3) Therefore, I dont know that Im looking at
  a piece of paper.
• (Skepticism)

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Skepticism

• In contemporary terminology, the ground for
  doubt proposed by Descartes can be put like this
  
• The Cartesian-style argument for Skepticism can
  now be put like this
• (1) If I know that Im looking at a piece of
  paper (A), then I know Im not dreaming (B).
• (2) I dont know that Im not dreaming (Not B)
• (3) Therefore, I dont know that Im looking at
  a piece of paper (Not A).
• (Skepticism)

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Skepticism

• In contemporary terminology, the ground for
  doubt proposed by Descartes can be put like this
  
• The Cartesian-style argument for Skepticism can
  now be put like this
• (1) If A then B
• (2) Not B
• (3) Therefore, Not A
• Modus Tollens

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Challenging the Argument

• There are always two options for challenging an
  argument.
• 1. Deny a premise
• 2. Deny the conclusion follows from the premises.
• Lets look at the premises.

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Premise 1

• (1) If I know that Im looking at a piece of
  paper (A), then I know Im not dreaming (B).
• Could we challenge 1?
• Could you know that youre looking at a piece of
  paper even if you dont know youre not asleep?
• Do you have to rule out the possibility that you
  are asleep before you can know youre looking at
  a piece of paper?

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Two Principles Supporting 1.

• Elimination of Relevant Alternatives
• We can't know P unless we've ruled out the
  possibilities we recognize to be incompatible
  with P.
• Closure
• If you know that P, and you know that P logically
  entails that Q, then you know Q too (or at least,
  you'll be in a position to know Q).

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Premise 2

• (2) I dont know that Im not dreaming (B)
• Some philosophers argue that you dont know
  youre not dreaming because your experiences
  dont guarantee that youre not dreaming.

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Russells Argument for Skepticism (according to
Moore)

• First, distinguish things you know immediately
  (what youre experiencing) from what you know
  inferentially (theres a piece of paper).
• 1. You dont know the paper exists immediately.
• 2. The existence of the paper doesnt follow
  logically from anything you know immediately.
• 3. So your belief concerning the paper must be
  based on analogical or inductive arguments.
• 4. What is so based cannot be certain knowledge.
• 5. What is so based cannot be knowledge.
• 6. So you dont know youre not dreaming.

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• Response We dont need to have a guarantee that
  something is true in order to know it. Suppose
  certainty is not required for knowledge.
• Is certainty required for knowledge?
• Perhaps not Are you certain of your name? No.
  But you still know your name.

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• (2) I dont know that Im not dreaming (B)
• A better argument for 2 is to point out that if
  you were having a certain type of dream, your
  experiences would be exactly as they are.
• Consider the hypothesis that you are dreaming you
  are in a philosophy class.
• So not only do you not know youre not dreaming,
  but you have evidence that you are dreaming (in a
  very particular way).

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• But perhaps you know youre not dreaming because
  the hypothesis that there is a piece of paper in
  front of you is the best explanation of your
  experiences.
• But why is it the best explanation?
• The most plausible?
• The simplest?

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Moores Proof of the External World

• Premise Here is one hand and here is another
• Conclusion The external world exists

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Moore as Giving a Modus Ponens Argument

• Skepticism
• (1) If I know that Im looking at a piece of
  paper (A), then I know Im not dreaming (B).
• (2) I dont know that Im not dreaming (Not B)
• (3) Therefore, I dont know that Im looking at
  a piece of paper (Not A).
• Moores Dogmatism
• (1) If I know that Im looking at a piece of
  paper (A), then I know Im not dreaming (B).
• (2) I know that Im looking at a piece of paper
  (A)
• (3) Therefore, I know Im not dreaming (B).

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• Skepticism
• (1) If (A), then (B).
• (2) (Not B)
• (3) (Not A).
• Moores Dogmatism
• (1) If (A), then (B).
• (2)(A)
• (3)Therefore, (B).

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Making Sense of Moore

• Moore must defend the premise that he knows he
  has hands.
• Moore Every argument must start with some
  premise. The more certain the premise the better.
• Few things are more certain than my belief that
  here is a hand.

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• In fact, (Moore speaking here) my belief I have a
  hand is more certain than any philosophical
  argument.
• It is more certain than either 1 (closure) or 2
  (I dont know Im not dreaming) or the belief
  that the conclusion follows.
• And it is certainly more certain than all three
  combined.

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The Feeling of Failure

• In argument, we want to convince our opponents
  that they are wrong.
• Moore fails to do this. To prove someone wrong,
  you can only use premises that your opponent
  accepts.
• The skeptic doesnt accept Here is a hand. So
  the skeptic wont be convinced.

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Explaining Moore

• Moore is not trying to convince the skeptic.
• He is just trying to show non-skeptics that they
  need not be convinced by the skeptics argument.
• The skeptic has a compelling argument, and Moore
  has a compelling argument.
• The result is a stand-off between Moore and the
  skeptic, with neither side changing their mind.

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Another argument by Moore

• Perhaps it is logically possible that I am having
  these exact experiences in a dream.
• But is it logically possible that I am having
  these exact experiences plus have these memories
  in a dream?
• This has not been shown to be logically possible.

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