Edmund Gettier

Edmund Gettier (born 1927) is an American philosopher who owes a substantial reputation to a single three-page paper published in 1963, called "Is Justified True Belief Knowledge?" In it Gettier questions the "JTB" definition of knowledge that was accepted by most philosophers at that time. Although the validity of this definition was arguably doubtful already in light of the work of Wittgenstein, among others, attention had not been drawn to this particular point.

Main article: Gettier problem

Gettier provides several examples of beliefs that are both true and justified, but that we should not intuitively call knowledge. Cases of this sort are now called "Gettier (counter-)examples." A typical one would run something like this. Suppose I go to my friend Alice's house and knock on the door, and I hear her say "Come in." I open the door and see her standing at the far end of the hallway, about thirty feet in front of me. At this point I might think to myself, and Believe "Alice is standing about thirty feet in front of me." In fact, Alice is standing about thirty feet in front of me, so my belief is True. Finally, I have a good deal of evidence for my belief: I see Alice, I heard her voice. Under normal circumstances we would say that I know that Alice is standing about thirty feet in front of me.

Now change the example a little. in fact, though I am unaware of it, what I see at the end of a the hall is a life-sized picture of Alice; and though I heard her voice coming from this direction it was actually coming through an intercom, so it gave me no evidence where she was really standing. Perhaps she is in the basement. In this case, my belief, though I was reasonably justified in holding it, turns out to be false. We should say that I don't know that Alice is about thirty feet in front of me.

Add one more change. Suppose that Alice's kitchen is right behind the poster at the end of the hall. She is in fact in her kitchen right now, and leaning against that wall. In this case I believe that she is about thirty feet in front of me, I have justification for that belief, and in fact it is true. Yet it would seem inaccurate to call my belief knowledge; it seems wholly accidental that Alice is there, and I would think she was there even if she weren't. Hence, Gettier claims, justified true belief is not in and of itself sufficient for knowledge.

Gettier inspired a great deal of work by philosophers attempting to recover a working definition of knowledge. Major responses include: Gettier's use of "justification" is too broad, and only some kinds of justification count; or, Gettier's examples do not count as justification at all, and only some kinds of evidence are justificatory; Knowledge must consist of justified true belief that is "truth-tracking"--belief held in such a way that if it turned out to be false it would not have been held, and vice versa.