The Problem of Skepticism

1
The Problem of Skepticism

• Can we really know anything?

2
Do you really know anything?

• Almost as long as there have been philosophers,
  there have been skeptics.
• Sometimes, skeptics are ridiculed as
  reactionaries - all they ever seem to do is say,
  You dont really know that, or, You cant
  really know that.
• But are there any good arguments for skepticism -
  any reasons why we should think we cant or dont
  know anything?

3
Varieties of Skepticism

• First, we should distinguish between two forms of
  skepticism.
• Local Skepticism denies that we can have
  knowledge of a particular category of beliefs.
• Global Skepticism denies that knowledge is
  possible at all.

4
Local Skepticism

• Examples of Local Skepticism are everywhere.
• Can we ever know what George Washington had for
  lunch on March 15, 1747?
• Other examples of Local Skepticism Can we ever
  really know whether an action is right or wrong?
• Can we ever know whether someone is really the
  same person from one day to the next?
• But while local skepticism is an interesting
  issue, global skepticism is much more of a threat
  to many philosophers.

5
Skepticism

• Setting aside local skepticism for the moment,
  what evidence could there be that global
  skepticism is true?
• An early global skeptic was Pyrrho of Elis (who
  lived around 300BCE).
• He was led to skepticism by considering two
  issues that had been long recognized by
  philosophers, and still are.

6
Appearance v. Reality

• The first issue is the recognition that the way
  things appear to us is not how they are in
  themselves.
• To see this, consider how things look at
  different angles and under different lighting
  conditions.
• Strictly speaking, well see the same
  mono-chromatic object as having different shapes
  and colors, even though we ignore these
  differences and believe that the object is one
  shape and one color.

7
More Examples

• Things that are far away appear smaller than they
  actually are.
• Objects reflected in mirrors are not real
  objects.
• Sticks look bent in water.
• Hallucinations.
• Dreams.

8
More Examples

• Objects appear differently under a microscope
  than they do to the naked eye.
• Your table looks flat, solid, and motionless.
  Yet physicists tell us that it is composed of
  molecules in constant motion, and that there is
  more space between these molecules than the mass
  of the molecules combined.

9
Relativity of Appearances

• This observation is combined with another that
  appearances are relative to the observer.
• For example, something that tastes sweet to one
  person tastes bitter to another something that
  looks red to a normal person looks brown to a
  color-blind person.
• But in themselves objects cannot be both bitter
  and sweet, or both red and brown.

10
Skepticism Perceptual Knowledge

• What these two issues - the difference between
  appearance and reality and the relativity of
  appearances - have in common is that they reveal
  a dogmatic belief that many of us have.
• We dogmatically assume that we have real
  knowledge of objects and their characteristics by
  means of perception.

11
Skepticism Perceptual Knowledge

• In fact, we uncritically assume that perception
  is our best (perhaps only) source of knowledge.
• But the skeptic denies that the beliefs we form
  on the basis of perception give us knowledge.
• Thats because the skeptic claims that we can
  never know whether or not our beliefs are true.

12
Skepticism Perceptual Knowledge

• Now, the skeptic does not deny that we form
  beliefs on the basis of perception.
• Nor does the skeptic deny that some of those
  beliefs might be true.
• What the skeptic denies is that we can never have
  any evidence that any of our perceptual beliefs
  are true.

13
Error Perceptual Belief

• One reason for claiming that we can never have
  any evidence that any of our perceptual beliefs
  are true comes from the difference between
  appearances and reality.
• Appearances sometimes mislead us into forming
  false beliefs about the way things actually are.
• In other words, appearances can lead to error.
• Hallucinations are an example of this.

14
Error Perceptual Belief

• The problem here is that, first, if were wrong
  about the way things are, we cannot say that we
  know how they are.
• Second, in many cases of perceptual error, we
  cannot distinguish between a false perceptual
  belief and a true perceptual belief.
• For example, someone who believes a hallucination
  has no way distinguishing between her perceptual
  belief and a true perceptual belief.

15
Error Perceptual Belief

• A similar problem arises due to the relativity of
  perception.
• The fact that one person sees an object as red
  and another as blue implies that at least one
  person is wrong - that someone has a false
  belief.
• But how could anyone determine whos belief is
  false and whos is true?

16
A Skeptical Argument

• For the skeptic, these considerations imply that
  there really isnt any evidence that a perceptual
  belief is true except another perception.
• But what evidence could we have that this second
  perceptual belief is true?
• The skeptic thinks we couldnt, and so in fact we
  dont have any evidence at all.

17
A Skeptical Argument

• We can use these considerations to formulate the
  following skeptical argument
• If some of our perceptual beliefs are erroneous,
  then it is always possible that any of our
  perceptual beliefs are erroneous.
• If it is always possible that any of our
  perceptual beliefs are erroneous, then we never
  know that any of our perceptual beliefs are true.
• Some of our perceptual beliefs are erroneous.
• Therefore, it is always possible that any of our
  perceptual beliers are erroneous.
• Therefore, we never know that any of our
  perceptual beliefs are true.

18
A Note on Logical Form

• The preceding argument is a combination of two
  instances of Modus Ponens, which has the
  following form
• If P then Q
• P
• Therefore, Q
• All arguments of this form are valid.

19

• The skeptics argument has the form of a compound
  modus ponens (MP) argument
• If P then Q
• If Q then R
• P
• Therefore, Q (MP applied to 1 and 3)
• Therefore, R (MP applied to 2 and 4)
• Any argument having this form is also valid. The
  question is whether or not it is sound.

20
A Skeptical Argument

• You might question the first premise of this
  argument.
• Why should we believe that just because we have
  some erroneous perceptual beliefs, it is always
  possible for any of our perceptual beliefs to be
  erroneous?
• We can see why by reconsidering hallucinations.

21
A Skeptical Argument

• When a person is hallucinating, the experiences
  she has could be exactly similar to the
  experiences she would have had were she not
  hallucinating.
• If thats so, then we cant say she has knowledge
  in the latter case but not in the former without
  being arbitrary.

22
A Skeptical Argument

• And thats just the skeptics point
• We have no evidence that would allow us to say
  that any perception is real rather than a
  hallucination.
• In other words, every perception you have could
  be a hallucination, and you cannot have evidence
  to determine when youre hallucinating and when
  youre not.

23
A Skeptical Argument

• If one of the goals of knowledge is to avoid
  believing what is false, then we should suspend
  our judgment about any belief until it is shown
  to be true.
• But the skeptic has argued that we can never have
  evidence that any perceptual belief is true, we
  must always suspend our judgment about them.

24
!Be Careful!

• Some may want to say, But I can know that I
  cant know anything!
• So far, the skeptic has claimed only that you do
  not have any evidence for your perceptual
  beliefs, and that therefore you must withhold
  your assent from any perceptual belief.
• This is a negative argument - an argument about
  what is not the case - rather than a positive
  argument for what is the case.

25

• The skeptic can assert that people lack knowledge
  without claiming to know that people lack
  knowledge - that is, without claiming to have
  evidence that the assertion that people lack
  knowledge is true.
• To attempt to provide such evidence would be to
  provide a positive argument for skepticism, which
  is not what we have.

26

• The mistake here is not the skeptics, but the
  anti-skeptics.
• If you think all truths are knowable, then you
  will think that the skeptical thesis isnt true.
• But why think that all truths are knowable?
• Why couldnt it be that there are many truths
  that we simply cannot know?