Knowledge and Justified True Belief

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Prior to Edmund Gettier, philosophers believed that knowledge was equivalent to justified true belief.  Since Plato, it had generally been agreed among philosophers that there are three criteria of propositional knowledge, individually necessary and jointly sufficient (Pryor, 2005; Cushing, 2000).  Before the Gettier philosophy, the following JTB Analysis (justification, truth, and belief) formed the basis of the theory of knowledge:

  • “S knows that P iff: P is true (truth criterion)
  • S believe that P (belief criterion)
  • S is justified in believing P (justification criterion)”

A classic example of the above proposition would be the one by Carl Ginet on fake barns.  A person is driving through rural Pennsylvania where there are a lot of fake barns: mere wooden fronts that look like barns from the road.  The person driving through is not aware of this and has no reason to suspect it.  As the person looks off to his or her right, and sees something that looks like a barn, then that person believes, “That’s a barn.”  As a matter of fact, it is a barn, as it is one of the few barns in the region which is not a fake.  But then that person would just be lucky.  If he or she had looked at a fake barn instead, then he or she would have believed that it was a barn (Pryor, 2005).

In this case then it would seem that the person’s belief that he or she drove by a barn is justified or reasonable simply because it looks like a barn and the person was not informed that the region was full of fake barns.  Then in this case, the person’s belief is also true.  But then the question is if that person knows that he or she is driving by a barn (Pryor, 2005). It seems then that justified true belief is not sufficient for knowledge.  It is this theory that Edmund Gettier is criticizing.


Gettier’s main objection is to the claim that justified true belief is sufficient for knowledge.  He presented examples in which the subject has a justified true belief which intuitively fails to count as knowledge (Pryor, 2005).  He does not question whether the three criterion are each necessary.  Rather, what Gather provides is that these propositions are not jointly sufficient.  In other words, Gettier provides that we can justifiably believe the true proposition P but not necessarily know P (Cushing, 2000).

In his philosophy, Gettier (1963) makes two important points.  First, the proposition wherein S is justified in believing P is a necessary condition of S’s knowing that P is open to the possibility that a person is justified in believing a proposition that is in fact false.

The second point is that for any proposition P, if S is justified in believing P, and P entails Q, and S deduces Q from P and accepts Q as a result of this deduction, then S is justified in believing Q (Gettier, 1963).   Stated differently, these two points represent two assumptions: 1) it is possible for someone to be justified in believing something false; and 2) if S is justified in believing P and P entails Q, then S is justified in believing Q (Cushing, 2000).

A classic Gettier example to illustrate these two points or assumptions would be the one about the Ford car.  Suppose a person called Smith has a justified belief  that someone in his office owns a Ford.  It is also true, as a matter of fact, that someone in the office does indeed own a Ford.  However, Smith’s evidence for his belief concerns Jones, from his office, who as it turns out does not own a Ford.  Smith’s belief that someone in the office owns a Ford is true because someone else in the office owns a Ford (not Jones).

The person who in fact owns a Ford is actually, for example, called Brown.  Yet all of Smith’s evidence concerns Jones, and not Brown, so it seems that intuitively, Smith doesn’t know that someone in his office owns a Ford.  It would seem then that Smith doesn’t know, even though Smith has a justified belief that someone owns a Ford, and as it turns out, this belief happens to be true (Pryor, 2005).

From the above example, it would seem that Smith has a justified belief in a true proposition (in that someone in his office owns a Ford), but this is not to say that he has knowledge of that proposition (since the owner of the Ford is Brown, not Jones, as Smith thought).   What Gettier (1963) thus tells is that even if the three criterion composed of truth, belief, and justification are individually necessary for knowledge, they are not jointly sufficient (Cushing, 2000).  This has been widely called as the Gettier Problem (Pryor, 2005; Cushing, 2000; Stanford Encyclopedia of Philosophy, 2006).

Thus, the JTB Analysis, previously mentioned as the existing proposition prior to the Gettier problem, does not state a sufficient condition for someone’s knowing a given proposition (Gettier, 1963).    In the example given on the Ford, the Gettier problem arises because of the proposition that a person knows that someone owns a Ford based on evidence that falls short of certainty.  If knowledge requires absolutely certain evidence, then the person Smith in the Ford example would not be in a position to know that someone owns a Ford .  His (Smith’s) evidence after all was not absolutely certain or infallible because he was mistaken as to who owned the Ford (Pryor, 2005).

Assuming that Gettier’s philosophy is correct, then a possible solution to the Gettier problem then would be that knowledge is justified true belief where the reasoning on which a person’s belief is based on does not proceed through any false steps or falsehood (Pryor, 2005).  However, the Gettier examples need not involve any inference, so there may be cases of justified true belief in which the subject fails to have knowledge although the S’s belief that P is not inferred from any falsehood.

The lesson from the Gettier problem then is that the justification condition by itself cannot ensure that belief that is true cannot be mistakenly identified as knowledge.  Even a justified belief (which is belief based on good evidence), can be true because of luck (Stanford Encyclopedia of Philosophy, 2006), such as the example on the Ford car wherein Smith’s belief that someone owns a car is true in the sense that someone (Brown) does indeed own a car, but Smith’s justified belief or good evidence as to the someone who owns the Ford actually pertains to someone else (Jones).

Assuming that Gettier is correct, a possible option for working out an account of what knowledge is.   Knowledge is justified true belief absent luck or accident.   Gettier’s fourth condition to knowledge (on the absence of falsehood) is not necessary as his cases indicate that a person can still hold on to a true belief based on luck or accident.  The third criterion in the JTB Analysis, on justification, itself requires that luck be excluded (Sudduth, 2005).  Thus, justified true belief may be sufficient for knowledge only if you eliminate luck or accident.


According to Gettier (1963), justified true belief can fail to constitute knowledge.  Justified true belief may not be sufficient for knowledge, and he further tells us that the three criterion of truth, belief, and justification are not jointly sufficient.  Gettier proposes a third condition, that true belief should not be based on any falsehood.  However, his philosophy involves the elements of luck or accident which allows the subject to hold on to a true belief.  Thus, it would seem that justified true belief may be sufficient for knowledge providing luck or accident are eliminated from the justification criterion.


  1. Cushing, Simon.  (2000).  Edmund Gettier: “Is Justified True Belief Knowledge?”  University of Michigan-Flint.  Retrieved November 1, 2006 from
  2. Gettier, Edmund L.  (1963).  Is Justified True Belief Knowledge?  Analysis 23: 121-123.  Transcribed into hypertext by Andrew Chrucky, September 13, 1997.  Retrieved November 1, 2006 from:
  3. Pryor, Jim.  (Spring 2004).  Theory of Knowledge – The Gettier Problem.  Princeton University.  Retrieved November 1, 2006 from:
  4. Sudduth, Michael.  (2005).  Justification and the Gettier Problem.  Dr. Michael Sudduth’s Philosophy Courses Webpage.  Retrieved November 1, 2006
  5. The Analysis of Knowledge.  (January 16, 2006). Stanford Encyclopedia of Philosophy.  Retrieved November 1, 2006 from:

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