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Gettier problem

The Gettier problem is a fundamental problem in contemporary epistemology (the philosophy of knowledge), issuing from counterexamples to the definition of knowledge as justified true belief.

The problem owes its name to a remarkable three-page paper published in 1963, by Edmund Gettier, called "Is Justified True Belief Knowledge?". In the paper, Gettier argues that it is not.


The JTB account

Until Gettier's essay was published, most analytic philosophers took it for granted that something we might call the JTB account of knowledge was correct. The JTB account claims that knowledge can be conceptually analyzed as justified true belief--which is to say that the meaning of sentences such as "Smith knows that it rained today" can be given with the following set of necessary and jointly sufficient conditions:

A subject S knows that a proposition P is true if, and only if:

  1. S believes that P
  2. P is true
  3. S is evidentially justified in believing that P is true

Gettier's paper used counterexamples to argue that there are cases of beliefs which are both true and justified--therefore satisfying all three conditions for knowledge on the JTB account--but which do not appear to be genuine cases of knowledge. Gettier, therefore, argued that his counterexamples show that the JTB account of knowledge is false--and thus, that a different conceptual analysis is needed to correctly track what we mean by "knowledge."

Gettier's counterexamples

Gettier's case is based on two purported counterexamples to the JTB analysis. Both of them rely on the fact that justification is preserved by entailment: that is, that if Smith is justified in believing P, and Smith realizes that the truth of P entails the truth of Q, then Smith would also be justified in believing Q. Gettier calls these counterexamples "Case I" and "Case II":

Case I

Smith has applied for a job, but has a justified belief that "Jones will get the job". He also has a justified belief that "Jones has 10 coins in his pocket". Smith therefore (justifiably) concludes (by the rule of the transitivity of identity) that "the man who will get the job has 10 coins in his pocket".
In fact, Jones does not get the job. Instead, Smith does. However, as it happens, Smith also has 10 coins in his pocket. So his belief that "the man who will get the job has 10 coins in his pocket" was justified and true. But it does not appear to be knowledge.

Case II

Smith has a justified belief that "Jones owns a Ford". Smith therefore (justifiably) concludes (by the rule of disjunction introduction) that "Jones owns a Ford, or Brown is in Barcelona", even though Smith has no knowledge whatsoever about the location of Brown.
In fact, Jones does not own a Ford, but by sheer coincidence, Brown really is in Barcelona. Again, Smith had a belief that was true and justified, but not knowledge.

False premises

In both of Gettier's actual examples, the justified true belief came about as the result of entailment from justified false beliefs. This led some early responses to Gettier to conclude that the definition of knowledge could be easily adjusted, so that knowledge was justified true belief that depends on no false premises.

More general Gettier-style problems

The "no false premises" (or "no false lemmas") solution was not the end of the matter, however, as more general Gettier-style problems have also been proposed, in which the justified true belief does not seem to be the result of a chain of reasoning from a justified false belief.

For example:

Smith enters a room and seems to see Jones. He immediately forms the justified belief that "Jones is in the room".
In fact, Smith did not see Jones, but rather, a lifelike replica. However, as it happens, Jones is in the room, though Smith has not seen him yet. Once again, Smith has a justified true belief that does not seem to be knowledge, yet this time it is more debatable whether he used any false premises in his reasoning.

Other responses to Gettier

Several more sophisticated responses have been offered to the Gettier problem; indeed, it would not be an overstatement to call the Gettier problem the single most important problem in contemporary Analytic epistemology. The different directions that responses have taken is dictated by the structure of Gettier's argument: if knowledge just is justified true belief, then there cannot be any cases of justified true belief that are not also cases of knowledge; but Gettier claims that his counterexamples are cases of justified true belief without being cases of knowledge. Therefore, we must either accept Gettier's conclusion--and come up with a new conceptual analysis for knowledge--or else we must deny one of Gettier's two claims about his counterexamples (that is, we must either deny that Gettier cases are justified true beliefs, or else we must accept that Gettier cases are knowledge after all).

Most contemporary epistemologists accept Gettier's conclusion. Their responses to the Gettier problem, therefore, consist of trying to find alternate analyses of knowledge. The most common direction for this sort of response to take is what might be called a "JTB+G" analysis: that is, an analysis based on finding some fourth condition—a "no-Gettier-problem" condition--which, when added to the conditions of justification, truth, and belief, will yield a set of necessary and jointly sufficient conditions. The "no false lemmas" condition is one such condition that was proposed early in the debate.

Another response is that of Alvin Goldman (1967), who suggests the addition of a causal condition: a subject's belief is justified, for Goldman, only if the truth of a belief has caused the subject to have that belief (in the appropriate way); and for a justified true belief to count as knowledge, the subject must also be able to "correctly reconstruct" (mentally) that causal chain. Goldman's analysis would rule out Gettier cases in that Smith's beliefs are not caused by the truths of those beliefs; it is merely accidental that Smith's beliefs in the Gettier cases happen to be true. Goldman faces the difficulty, however, of giving a principled explanation of how an "appropriate" causal relationship differs from an "inappropriate" one (without the circular response of saying that the appropriate sort of causal relationship is the knowledge-producing one). Thus, adopting a causal response to the Gettier problem usually requires one to adopt (as Goldman gladly does) some form of reliabilism about justification.

Closely related, but without quite the same theoretical baggage, is Robert Nozick's subjunctive account (1981), on which knowledge is a belief which is true, and which the believer would have so long as it is true, and would not have had if it had been false. Nozick's reformulation is intended to preserve Goldman's intuition that Gettier cases should be ruled out by ruling out "accidentally" true justified beliefs, but without requiring the burly consequences of building a causal requirement into the analysis. Of course, this move has prompted the accusation that Nozick's account merely hides the problem rather than solving it--for it leaves open the question of why Smith would not have had his belief if it had been false. The most promising answer seems to be that it is because Smith's belief was caused by the truth of what he believes; but that puts us back in the causalist camp.

Keith Lehrer and Thomas Paxson (1969) proposed another commonly cited response, by adding a defeasibility condition to the JTB analysis. On their account, knowledge is undefeated justified true belief -- which is to say that a justified true belief counts as knowledge if and only if it is also the case that there is no further truth which, had the subject known it, would have defeated her present justification for the belief. (Thus, for example, Smith's justification for believing that the person who will get the job has ten coins in his pocket is his justified belief that Jones will get the job, combined with his justified belief that Jones has ten coins in his pocket. But if Smith had known the truth that Jones will not get the job, that would have defeated the justification for his belief.) However, many critics (such as Marshall Swaine [1974]) have argued that the notion of a defeater fact cannot be made precise enough to rule out the Gettier cases without also ruling out legitimate cases of knowledge.

The difficulties involved in producing a fourth condition have led some to believe that trying to fix the JTB account is a dead-end game. For example, one might argue that what the Gettier problem shows is not the need for a fourth independent condition in addition to the original three, but rather that the attempt to build up an account of knowledging by conjoining a set of independent conditions was misguided from the outset. Those who take this take generally argue that epistemological terms like justification, evidence, certainty, etc. should be analyzed in terms of a primitive notion of knowledge, rather than vice versa. Knowledge is understood as factive, that is, as embodying a sort of epistemological "tie" between a truth and a belief. The JTB account is then criticized for trying to get the factivity of knowledge "on the cheap," by replacing an irreducible notion of factivity with the conjunction of some of the properties that accompany it (in particular, truth and justification). Of course, the introduction of irreducible primitives into a philosophical theory is always a controversial move (some would say a sign of desperation), and such anti-reductionist accounts are unlikely to please those who have other reasons to hold fast to the method behind JTB+G accounts.

Finally, one might respond to Gettier by finding a way to avoid his conclusion in the first place. However, it can hardly be argued that knowledge is justified true belief if there are cases that are justified true belief without being knowledge; thus, those who want to avoid Gettier's conclusion have to find some way to defuse Gettier's counterexamples. In order to do so, they must either accept (1) that Gettier's cases are not really cases of justified true belief; or (2) that Gettier's cases really are cases of knowledge after all.

Since Gettier's cases stipulate that Smith has a certain belief and that his belief is true, it seems that in order to propose (1), one must argue that Gettier goes wrong because he has the wrong notion of justification. Such an argument often depends on an externalist account on which "justification" is understood in such a way that whether or not a belief is "justified" depends not just on the internal state of the believer, but also on how that internal state is related to the outside world. Externalist accounts typically are constructed such that Smith's beliefs in Case I and Case II are not really justified (even though it seems to Smith that they are), because his beliefs are not lined up with the world in the right way. Such accounts, of course, face the same burden as causalist responses to Gettier: they have to explain what sort of relationship between the world and the believer counts as a justificatory relationship.

Those who accept (2) are by far in the minority in Anglo-American philosophy; generally those who are willing to accept it are those who have independent reasons to say that more things count as knowledge than the intuitions that led to the JTB account would acknowledge. Chief among these are epistemic minimalists such as Crispin Sartwell , who hold that all true belief, including both Gettier's cases and lucky guesses, counts as knowledge.

External links


  • Edmund Gettier: Is Justified True Belief Knowledge? in Analysis, v. 23. Available at
  • Alvin Goldman: A Causal Theory of Knowing in The Journal of Philosophy v. 64 (1967), pp. 335-372.
  • Keith Lehrer and Thomas Paxson: Knowledge: Undefeated Justified True Belief, in The Journal of Philosophy v. 66 (1969), pp. 1-22.
  • Robert Nozick: Philosophical Explanations. Harvard University Press. 1981.
  • Marshall Swain: Epistemic Defeasibility, American Philosophical Quarterly, v. II n. 1 (January 1974).

Last updated: 05-07-2005 12:18:19
Last updated: 05-13-2005 07:56:04