Justified True Beliefs Knowledge

1
Justified True Beliefs Knowledge?
2
Justified True Beliefs?

• Descartes and Locke agree that justified true
  beliefs are identical to knowledge, although they
  disagree how knowledge is ultimately justified.
• Whether knowledge is justified by reason or
  experience, justification is considered to be a
  psychological process--whether a person is
  justified in believing something depends on what
  she is conscious of.
• But does this conception of knowledge corresponds
  to our common sense?

3
Justified True Beliefs?

• In fact, this conception has been challenged in
  two ways
• Are all justified true beliefs knowledge?
• To ask this question is to ask whether (i) being
  true and (ii) being justified form a sufficient
  condition for a belief to be knowledge.
• Is all knowledge justified true beliefs?
• To ask this question is to ask whether (i) being
  true and (ii) being justified are necessary
  conditions for a belief to be knowledge.

4
Some justified true belief is not knowledge

• Consider the 1st question Are all justified true
  beliefs knowledge?
• Suppose You have seen me putting a 10 coin into
  my pocket and believe that I have (at least) 10
  in my pocket.
• According to common sense, under normal
  situations, your perception provides good
  justification for your belief.
• So you have a justified true belief that I have
  10 in my pocket.
• So , under normal situations, you know that I
  have 10 in my pocket.

5
Some justified true belief is not knowledge

• However suppose further that my pocket has a hole
  and the coin has dropped out of my pocket through
  the hole.
• However, there is still a 10 note in my pocket
  you have not seen. So your belief that I have 10
  in my pocket is still true.
• Your belief is justified because you have the
  same justification as in normal situations.
• Therefore, you still have justified true belief
  that I have 10 in my pocket.

6
Some justified true belief is not knowledge

• However, common sense tells us that you do not
  know that I have 10 in my pocket.
• This shows that being true and being justified
  are not sufficient for a belief to be knowledge.

7
Objection

• Some argue that you are not justified to believe
  that I have 10 in my pocket because you infer
  this conclusion from a false premise, i.e., I
  have a 10 coin in my pocket.
• No conclusion is justified if it is inferred from
  a false premise.
• We should add this no-false-premise-principle to
  the criterion of justification.

8
Response

• The addition of no-false-premise-principle cannot
  exclude all counterexamples.
• Suppose You have seen me putting a fake 10 coin
  into my pocket (although you do not know that
  that is a fake one) and believe that I have 10
  in my pocket.
• I, coincidentally, have a 10 coin in my pocket.

9
Response

• In this case, your belief is inferred from a true
  premise, i.e., I have a 10 coin in my pocket.
• But common sense tells us that you still do not
  know that I have (at least) 10 in my pocket.
• The above cases show that for a justified true
  belief to be knowledge, the beliefs
  justification must be related to what makes the
  belief true.

10
Some knowledge is not justified true belief

• Consider the 2nd question Is all knowledge
  justified true beliefs?
• You believe that the sum of interior angles of a
  triangle is equal to 2 right angles.
• However, you forget how to do a geometrical proof
  for this, even though you knew how to do it in
  the past.
• Moreover, you would understand the proof if you
  see it again.

11
Some knowledge is not justified true belief

• Consider a child who also believe that the sum of
  interior angles of a triangle is equal to 2 right
  angles.
• He believes this because this is what his friend
  told him. But this is a pure guess of his friend.
• He would not understand the proof if it is
  presented to him.
• According to common sense, you, but not the
  child, know that the sum of interior angles of a
  triangle is equal to 2 right angles.

12
Some knowledge is not justified true belief

• So, although you cannot justify your belief,
  according to common sense, you still know that
  the sum of interior angles of a triangle is equal
  to 2 right angles.
• This shows that being being justified are not
  necessary for a belief to be knowledge.

13
Conclusion

• So being true and being justified are neither
  sufficient nor necessary for a belief to be
  knowledge in our commonsense.
• I.e. justified true belief is not the
  definition of knowledge.
• But the fact that we do not know the definition
  of knowledge does not imply that we dont know
  anything.