Relevant Alternatives

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Relevant Alternatives

• Elusive Knowledge

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David Lewis

• 1941-2001
• Student of Quine.
• Has a ground-breaking paper
• in every area of philosophy.
• Common-sense based (except for that possible
  worlds thing).
• Bit weird.

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First pass

• S knows P if and only if Ss evidence eliminates
  every possibility in which not-P.
• Evidence Perceptual experience plus memory.
• Example S knows the wall is white if and only if
  Ss evidence eliminates every possibility in
  which the wall is not white.

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• Suppose there are two possibilities
• Then you have an experience of a white wall.

The wall is white
The wall is black
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• S knows P if and only if Ss evidence eliminates
  every possibility in which not-P.

The wall is white
The wall is black
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Objection
The wall is white and it looks white
The wall is black and it looks black
Accurate senses

• Hang on, youre assuming youre
• not being deceived.

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Objection
The wall is white and it looks white
The wall is black and it looks black
Accurate senses
The wall is black and it looks white
The wall is white and it looks black

• Hang on, youre assuming youre
• not being deceived.

Deceived
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Final Pass

• S knows P if and only if Ss evidence eliminates
  every possibility in which not-P
• - except for those possibilities we are properly
  ignoring.

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Objection
The wall is white and it looks white
The wall is black and it looks black
Accurate senses
Relevant alternatives
The wall is black and it looks white
The wall is white and it looks black
Deceived
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Analogy

• A surface is flat if and only if there are no
  bumps on it.
• Objection Nothing is flat, as there are always
  tiny bumps.
• A surface is flat if and only if there are no
  bumps on it except for those bumps we are
  properly ignoring.

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Presuppositions

• Every conversation has presuppositions.
• We may presuppose that certain alternatives are
  false.
• We may presuppose that we are not being deceived
  by an evil demon.

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Which alternatives are relevant?

• Must not be ignored
• 1. Actual
• 2. Believed
• 3. Resemblance
• 7. Rule of attention
• Can be ignored
• 4. Rule of reliability
• 5. Rule of method
• 6. Rule of conservatism

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1. Rule of actuality

• The actual world is always relevant.
• If the Canucks will lose, we cannot presuppose
  theyll win.
• We cannot know anything by ignoring the
  possibility (actuality) that the Canucks will
  win.
• We cant know that the victory party will be
  cancelled (by ignoring the possibility of a win).

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2. Rule of belief

• Nothing that is (or ought to be) believed can be
  ignored.
• If I believe Santa exists, then I cannot ignore
  the possibility that Santa exists.
• I cannot know that Billy wont get presents
  unless his parents buy them
• even if my evidence eliminates every other
  possibility.

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2. Rule of Belief

• Nothing that is (or ought to be) believed can be
  ignored.
• Suppose I have good evidence that Santa exists,
  but (irrationally) refuse to form the belief.
• I cannot know that Billy wont get presents
  unless his parents buy them
• even if I dont believe in Santa.

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Degree of belief

• A belief in an alternative can destroy knowledge.
• To what degree must the belief be held?
• I know the defendant is guilty
• My degree of belief that a well-trained dog
  committed the murder is very low.
• If the degree of belief were higher,
• I wouldnt know.
• How much higher?
• Depends on context.

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3. Rule of resemblance

• A possibility that saliently resembles a relevant
  possibility may not be ignored.

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Lottery problem

• You dont know the ticket will lose the lottery.
  Why not?
• For every ticket, there is a possibility that it
  will win. They are all similar. So either they
  can all be ignored or none of them. One of them
  is actual. So they cannot be ignored.

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Cannot be ignored by the rule of actuality
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Cannot be ignored by the rule of actuality
Cannot be ignored by the rule of resemblance
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Resemblance explains Gettier cases

• The clock is stuck at noon.
• I look at it at 1200 and pick up a justified
  true belief that isnt knowledge. Why not?

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• It isnt knowledge because I have not eliminated
  the possibility that I looked at the clock at
  1210.
• This possibility resembles actuality perfectly as
  far as the clock goes.

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• I see a real barn while driving through fake-barn
  country.
• I dont know it is a barn. Why not?
• Because I cannot ignore the possibility that I am
  seeing a fake barn.
• This possibility resembles actuality as far as
  the abundance of fake barns goes.

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We must apply the rule of resemblance with care

• Can I ignore the possibility that a demon is
  deceiving me?
• This possibility resembles actuality as far as my
  experiences go!
• The rule of resemblance was never meant to apply
  to this resemblance !!

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7. Rule of attention

• You cannot ignore a possibility you are attending
  to.
• When a sceptic makes you attend to a sceptical
  hypothesis, it becomes relevant.
• This accounts for the effects Dretske describes.
• If you dont think the sceptic makes the
  hypothesis relevant, you dont need this rule.

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4. Rule of reliability

• We may ignore possibilities in which usually
  reliable processes fail.
• We can ignore the possibility that my eyes are
  misleading me on this occasion.

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5. Rule of method

• We can suppose a sample is representative.
• If we draw 50 red balls from an urn, we can
  ignore the possibilities in which the sample is
  not representative.
• Defeasible

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6. Rule of conservatism

• If everyone uses a certain rule of inference, you
  may ignore possibilities in which that rule is
  misleading.
• Respect your elders.
• Very defeasible.