The traditional account of knowledge is as follows:

  1. P is true (truth)
  2. S believes that P (belief)
  3. S is justified in believing that P (justification)

For instance, we can say that I know that it snowed if and only if:

  • It really snowed, and
  • I believe that it snowed, and
  • I am justified in believing that it snowed (I saw snowy trees, snow on the ground).

So, we have 3 individually necessary and jointly sufficient conditions: truth, belief and justification. If they are met of a given claim, then we have knowledge of the claim. In other words, my knowledge is true justified belief.

Edmund Gettier showed that these 3 conditions are not sufficient for someone’s knowing a given proposition. Suppose that someone called Smith (S) has a strong evidence for the following proposition: Jones owns a Ford (P). Smith’s evidence might be that Jones has at all times in the past within Smith’s memory owned a car, and always a Ford. Smith has another friend, Brown, and Smith is totally unaware of where Brown is now. Smith selects randomly a place (Barcelona) and constructs the following proposition (P): “Either Jones owns a Ford, or Brown is in Barcelona.

The proposition “Jones owns a Ford” (P) is transformed into “Jones owns a Ford or Brown is in Barcelona” ( P v Q ) by deductively valid inference, where (Q) is added as a disjunct.

P → P v Q 

Imagine that two further conditions hold. First, Jones does not own a Ford, but rents it. Second, by coincidence, Brown is in Barcelona. If these two conditions hold, then Smith does not know that P is true, even though:

  1. P is true (Brown is in Barcelona)
  2. S believes that P
  3. S is justified in believing that P is true (Smith has correctly inferred P from a proposition for which he has a strong evidence)

Thus, these 3 conditions are no sufficient for someone’s knowing a given proposition.

Bibliography:

  • Gettier, Edmund L. (1963). “Is Justified True Belief Knowledge?”. Analysis. 23 (6): 121–123.