Spring 2004

Theory of Knowledge The Gettier Problem

Assoc. Prof. James Pryor

Suppose you're driving through rural Pennsylvania. As a matter of fact, the region you're driving through contains a lot of fake barns: mere wooden fronts that just look like barns from the road. But you don't know this, and have no reason to suspect it. You look off to your left and you see something that looks like a barn, so you believe "That's a barn." In fact, it is a barn. It's one of the few barns in the region. But you're just lucky. If you had looked at a fake barn instead, you would have believed that it was a barn.

Cases of this sort are known as Gettier cases, after the philosopher Edmund Gettier. Before Gettier, philosophers thought that knowledge was equivalent to justified true belief. That is:

But then Gettier came along and presented examples in which the subject has a justified true belief which, intuitively, fails to count as knowledge. The fake barn case we just discussed is an example of that sort. (That particular example was created by Carl Ginet). Here's another Gettier case:

You have a justified belief that someone in your office owns a Ford. And as it happens it's true that someone in your office owns a Ford. However, your evidence for your belief all concerns Nogot, who as it turns out owns no Ford. Your belief that someone in the office owns a Ford is true because someone else in the office owns a Ford. Call this guy Haveit. Since all your evidence concerns Nogot and not Haveit, it seems, intuitively, that you don't know that someone in your office owns a Ford. So you don't know, even though you have a justified belief that someone owns a Ford, and, as it turns out, this belief happens to be true.

These cases are counter-examples to the claim that justified true belief is sufficient for knowledge.

The Gettier Problem is to state what, in addition to or instead of justified true belief, is needed to have knowledge.

1. Notice that the Gettier Problem only arises because we were trying to say that you could know that someone owns a Ford on the basis of evidence that falls short of certainty. If we instead said that knowledge requires infallible or absolutely certain evidence, then it would be clear why you're not in a position to know that someone owns a Ford. You don't have infallible evidence that someone owns a Ford.

   This solution would do the trick. It would make our account of knowledge immune to Gettier-type counterexamples. But it would also make it next to impossible to have knowledge. So it seems like a bit of overkill. Most philosophers who have attacked the Gettier Problem have tried to find some solution which still allows subjects to know things on the basis of fallible, defeasible evidence.

2. One salient feature of the Nogot/Haveit Gettier case is that the reasoning that leads you to the belief that someone owns a Ford goes through a false step, namely the step where you believe that Nogot owns a Ford. So a possible solution to the Gettier Problem might be this: knowledge is justified true belief--where the reasoning your belief is based on doesn't proceed through any false steps.

   Philosophers initially thought this was a promising solution. But unfortunately, Rich Feldman described Gettier-like cases where your reasoning doesn't proceed through any false steps, but intuitively you still don't count as knowing. So the present solution doesn't get to the root of the problem.

   Feldman's case works like this:

   You see Nogot waxing a Ford, humming Ford ad jingles to himself, and so on. On that basis, you conclude that someone in your office is waxing a Ford, humming Ford ad jingles, and so on. This belief is true. On the basis of that belief, you conclude that someone in your office owns a Ford. As before, Haveit owns a Ford so this belief is true.

   Once again it seems like you have a justified true belief that someone owns a Ford, which fails to count as knowledge. In this case it does not look like your reasoning proceeded through any false steps.

   Another kind of Gettier case where you don't seem to reason through any false steps is discussed in the Rosenberg dialogue:

   You're in the meadow, and you see a rock which looks to you like a sheep. So you say to yourself "There's a sheep in the meadow." In fact there is a sheep in the meadow (behind the rock, where you can't see it).

   Here again you have a justified true belief that there is a sheep in the meadow, which fails to count as knowledge. In this case, your belief doesn't seem to be based on any reasoning at all. (You could argue the same for the original example with the barn, too.)

3. Another popular solution to the Gettier Problem is to say that you know that P iff you have a justified true belief that P, and there's no true information "out there" in the world that would defeat your justification for P, were you to learn of it.

   This sounds good at first. In the Nogot/Haveit case, there is the information that Nogot is merely pretending to own a Ford. If you learn that information, that would defeat your justification for believing that someone in the office owns a Ford. So according to the present proposal, even before you learn that information, the mere fact that it is "out there" in the world shows that your justified belief that someone owns a Ford can't count as knowledge (even though it happens to be true).

   In Rosenberg's sheep-in-the-meadow case, there is the information that what you're looking at is a rock. If you learn that information, that would defeat your justification for believing that there is a sheep in the meadow. So according to the present proposal, even before you learn that information, the mere fact that it is "out there" in the world shows that your justified belief that there is a sheep in the meadow can't count as knowledge (even though it happens to be true).

   In the fake barn case, there is the information that the region you're driving through has a lot of fake barns in it, and they look just like real barns. If you learn that information, that would defeat your justification for believing that you're looking at a real barn. Hence, even if it's true that you're looking at a real barn, according to the present proposal you don't know that you are.

   So far, this all sounds good. The present proposal also explains why you can't have knowledge in another sort of case, first introduced by Gilbert Harman:

   Jill reads in the newspaper that the president of her country has been assassinated. In fact, this story is true. However, the president's associates have mounted a campaign to suppress the story, and they've been broadcasting false reports on all the television stations that the president is OK, the assassin actually only killed a bodyguard. Jill is blissfully unaware of all this misleading evidence. The newspaper she read happens to be the only news source that's reporting the true events. All of Jill's peers, on the other hand, have heard the misleading TV reports and aren't sure whether or not the president was really killed.

   Harman claims that this is another Gettier case: he says that Jill has a justified true belief that the president was assassinated, but she doesn't have knowledge, because there is all this misleading evidence abroad in her community, which she has only managed to avoid by sheer luck.

   So far, so good. However, the current proposal also runs into difficulties.

   Do we really want to say that if there's any potentially defeating piece of information out there, that's enough to block your justified true belief from counting as knowledge? Consider a variant of Harman's assassination case.

   The president's associates are sitting in the TV studio, saying into the microphone "No really the president is OK, it was somebody else who got killed." The fact that they are saying this is a potentially defeating piece of information. If you were to learn that the president's associates are saying this, it would defeat your justification for believing the president had been assassinated. But suppose that in this case, unlike the earlier case, the associates' speech never gets broadcast to the public. (Maybe they're just joking around before the TV cameras start rolling.) So all the newspapers and TV stations carry the correct report about the president's assassination.

   In this case, it seems like you should know. There is a potentially defeating piece of evidence out there (the fact that the associates are saying what they're saying). But it's so remote, nobody knows about it except the associates themselves and a few TV crew. Perhaps this case is hard to assess. But my own inclination is to say that in this case, you can know the president was assassinated. The information about the president's associates in this case should not block your justified true belief about the president from counting as knowledge.

   Here's another case, to make the same point.

   You see Tom Grabit hide a book underneath his jacket and sneak out of Widener Library. On the basis of this, you form the justified belief that Tom stole a library book. As it happens, your belief is true. However, unbeknownst to you, Tom's mother was going around today telling people that Tom was thousands of miles away, and that Tom's evil twin John was visiting Harvard. The fact that Tom's mother said this is a potentially defeating piece of evidence. If you were to learn of it, it would defeat your justification for believing that Tom stole the book. However, as it turns out, it really was Tom who stole the book. Tom has no twin brother and his mother is a compulsive liar.

   In this case, too, it seems like you should count as knowing that Tom stole the book. The testimony of a compulsive liar, locked up in an asylum somewhere, which you never hear, should not block your justified true belief about Tom from counting as knowledge.

   In both of the preceding two cases, it seemed intuitively like you should count as having knowledge. Yet in both cases, there was some potentially defeating piece of information "out there" in the world. So the present proposal would say that you don't have knowledge. That is a problem for this proposal.

   You may have noticed that, in the preceding two cases, although there is some potentially defeating piece of information "out there," you would only be misled by that information if you somehow managed to learn only part of the truth. The information about what the president's associates are saying would only defeat your justification if you were ignorant of the fact that they were doing a cover-up. The information about what Tom's mother said would only defeat your justification if you were ignorant of the fact that she was a compulsive liar. And so on. So you might be tempted to say:

   Hey, we can fix the present proposal. We'll say that you know that P iff you have a justified true belief that P, and there's no true information "out there" in the world that would defeat your justification for P, were you to learn of it--unless there's also some second piece of information that would counteract that defeater.

   If you thought of this, that's good, you're on your toes. That's very clever.

   But if you think harder, you'll realize it's not clever enough. For if we amend the proposal in that way, then the proposal becomes incapable of explaining any of the Gettier cases. For in a Gettier case, your belief always happens to be true, and that's a piece of information "out there" in the world that would counteract any evidence that tells against your belief.

   To illustrate this, consider the original Nogot/Haveit case. In that case, you have a justified belief that someone owns a Ford, and this belief happens to be true. Now, there is some information out there--the information that Nogot is only pretending to own a Ford--that would defeat your justification for believing that someone owns a Ford. But, since your belief that someone owns a Ford is true, there is also a second piece of information out there that would counteract that defeater. Namely, the information that Haveit does own a Ford. If you were to learn both pieces of information--that Nogot is only pretending but Haveit does own a Ford--then you would remain justified in believing that someone owns a Ford. So according to the amended proposal, you would count as having knowledge. But that's the wrong result. In the Nogot/Haveit case, intuitively, you don't know that someone owns a Ford.

   So it is difficult to explain why in some cases (the Nogot/Haveit case, the sheep-in-the-meadow case, the Gadwall duck/grebe case, Harman's original assassination case) the existence of some defeating evidence "out there" in the world blocks your justified belief from counting as knowledge, even though it is true; and why in other cases (the second version of the Harman case, the Tom Grabit case) it does not.

   Sigh. Back to the drawing board.

As you can see, it's very difficult to say what we need to add to justified true belief, to turn it into knowledge. We'll come back to this problem again in a few classes.

In the meantime, let me call your attention to another lesson we can learn from all these Gettier cases. When we were discussing Austin, we saw him saying that you don't need to rule out all the alternatives to P, to know that P. You just need to rule out the alternatives that are somehow "relevant." Austin understood that to mean: you only need to rule out those alternatives you've encountered some evidence for.

We can model Gettier cases with the Relevant Alternatives Theory too. But this puts a new twist on the Theory. Consider the fake barn case. You really do see a barn, but we think you fail to know it's a barn, because you aren't in a position to rule out the possibility that it's a fake barn. It doesn't matter that you don't know about the prevalence of fakes hereabouts. The mere fact that there are a lot of fakes around seems to be enough to make the fake barn possibility a relevant one: a possibility you need to rule out, if you're to know you're looking at a real barn.

So reflection on Gettier cases seems to show that alternatives can be relevant--can be such that your inability to rule them out keeps you from having knowledge--merely because your environment is a certain way, regardless of whether you have any evidence that it's that way.

This theme is central to Dretske's use of the Relevant Alternatives Theory, which we'll look at next.

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