Gettier problem

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The Gettier problem is a philosophical question about whether a piece of information that happens to be true but that someone believes for invalid reasons, such as a faulty premise, counts as knowledge. It is named after American philosopher Edmund Gettier, who wrote about the problem in a three-page paper published in 1963, called "Is Justified True Belief Knowledge?". The paper refers to the concept of knowledge as justified true belief (JTB), credited to Plato. In the paper, Gettier proposed two scenarios where the three criteria (justification, truth, and belief) seemed to be met, but where the majority of readers would not have felt that the result was knowledge due to the element of luck involved.

The term is sometimes used to cover any one of a category of thought experiments in contemporary epistemology that seem to repudiate a definition of knowledge as justified true belief.

The responses to Gettier's paper have been numerous. While some rejected Gettier's examples, many sought to adjust the JTB account of knowledge to lessen the impact of both Gettier's own problems and other problems (collectively titled "Gettier problems") created in their mould. Since 1963, experiments have also been conducted to determine whether the instinctive reactions of those presented with a Gettier problem are uniform or display language or genetic biases.


The question of what the conditions are that must be met for something to really qualify as "knowledge" is as old as philosophy itself, and is a constant theme of Plato's dialogues, notably Meno 97a-98b. The more specific problem Gettier raises was also raised by Bertrand Russell in The Problems of Philosophy.[1] In Russell’s stopped clock case, as modified by Israel Scheffler,[2] Alice sees a clock that reads two o’clock. She believes it’s two o’clock, and that is true. However, unknown to Alice, the clock she’s looking at stopped twelve hours ago. So, she has an accidentally true, justified belief. Russell provides an answer of his own to the problem. Edmund Gettier's formulation of the problem was important as it coincided with the rise of the sort of philosophical naturalism promoted by W.V.O. Quine and others, and was used as a justification for a shift towards externalist theories of justification.[1] John L. Pollock and Joseph Cruz have stated that the Gettier problem has "fundamentally altered the character of contemporary epistemology" and has become "a central problem of epistemology since it poses a clear barrier to analyzing knowledge".[3]

Alvin Plantinga rejects the historical analysis:


Despite this, Plantinga does accept that some philosophers before Gettier have advanced a justified true belief account of knowledge, specifically C.I. Lewis and A.J. Ayer.[4]

Knowledge as justified true belief[edit]

The JTB account of knowledge is the claim 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 sufficient conditions:

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

Gettier's counterexamples[edit]

Gettier's paper used counterexamples (see also Thought Experiment) to argue that there are cases of beliefs that are both true and justified—therefore satisfying all three conditions for knowledge on the JTB account—but that 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 case is based on two counterexamples to the JTB analysis. Both of them rely on the established claim (under JTB) that justification is preserved by entailment, and the further claim that such applies significantly, or can be applied there coherently to the "stipulation" attributed to Smith's putative "belief" in the case of this particular counter-example: 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[edit]

Smith has applied for a job, but, it is claimed, 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 (unknowingly and by sheer chance) also had 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[edit]


False premises[edit]


More general Gettier-style problems[edit]

In an 1966 scenario known as 'The sheep in the field' Roderick Chisholm asks us to imagine that someone is standing outside a field looking at something that looks like a sheep (although in fact it is a dog disguised as a sheep). They believe there is a sheep in the field, and in fact they are right because there is a sheep behind the hill in the middle of the field. Hence, they have a justified true belief that there is a sheep in the field. But is that belief knowledge? A similar problem which seeks to be more plausible called the 'Cow in the Field' appears in Martin Cohen's book 101 Philosophy Problems, where it is supposed that a Farmer checking up on his favourite cow, confuses a piece of black and white paper caught up in a distant bush for his cow. However, since the animal actually is in the field, but hidden in a hollow, again, the Farmer has a justified, true belief which seems, nonetheless, not to qualify as 'knowledge'. Another scenario by Brian Skyrms is 'The Pyromaniac,[5] in which a struck match lights not for the reasons the pyromaniac imagines but because of some unknown 'Q radiation'. A different perspective on the issue is given by Alvin Goldman in the 'fake barns' scenario (crediting Carl Ginet with the example).[6] In this one, a man is driving in the countryside, and sees what looks exactly like a barn. Accordingly, he thinks that he is seeing a barn. In fact, that is what he is doing. But what he does not know is that the neighborhood generally consists of many fake barns — barn facades designed to look like exactly like real barns when viewed from the road. Since if he had been looking at one of them, he would have been unable to tell the difference, his 'knowledge' that he was looking at a barn would seem to be poorly founded .

The "no false premises" (or "no false lemmas") solution which was proposed early in the discussion, proved to be somewhat problematic, as more general Gettier-style problems were then constructed or contrived, 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:

After arranging to meet with Mark for help with homework, Luke arrives at the appointed time and place. Walking into Mark's office Luke clearly sees Mark at his desk; Luke immediately forms the belief 'Mark is in the room. He can help me with my logic homework'. Luke is justified in his belief; he clearly sees Mark at his desk. In fact, it's not Mark that Luke saw; it was a marvelous hologram, perfect in every respect, giving the appearance of Mark diligently grading papers at his desk. Nevertheless, Mark is in the room; he is crouched under his desk reading Frege. Luke's belief that Mark is in the room is true (he is in the room, under his desk) and justified (Mark's hologram is giving the appearance of Mark hard at work).

Again, it seems as though Luke does not "know" that Mark is in the room, even though it is claimed he has a justified true belief that Mark is in the room, but it's not nearly so clear that the perceptual belief that "Mark is in the room" was inferred from any premises at all, let alone any false ones, nor led to significant conclusions on its own; Luke didn't seem to be reasoning about anything; "Mark is in the room" seems to have been part of what he seemed to see.

To save the "no false lemmas" solution, one must logically say that Luke's inference from sensory data does not count as a justified belief unless he consciously or unconsciously considers the possibilities of deception and self-deception. A justified version of Luke's thought process, by that logic, might go like this:

  1. That looks to me like Mark in the room.
  2. No factor, right now, could deceive me on this point.
  3. Therefore, I can safely ignore that possibility.
  4. "Mark is in the room," (or, 'I can safely treat that as Mark.')

And the second line counts as a false premise. But by the previous argument, this suggests we have fewer justified beliefs than we think we do.

In another example, Matthew drives through an area that appears to have many barns. In fact it contains a great many realistic barn facades, perhaps made to help shoot a Hollywood movie 'on location'. When Matthew looks at the one real barn along his route, he forms the allegedly justified true belief, 'There's a barn over there.' But if he follows the strong requirement for justified belief, then his thought process will follow the previous mentioned steps exactly. A similar process appears in Robert A. Heinlein's Stranger in a Strange Land as an example of "Fair Witness" behavior.

Other responses to Gettier[edit]

The Gettier problem is posed in terms of a problem in first-order logic, but the introduction into the discussion by Gettier of terms such as belief and knows moves the discussion into the field of epistemology. Here, the sound (true) arguments ascribed to Smith, then need also to be valid (believed) and convincing (justified) if they are to issue in real-world discussion about justified true belief. [7] Gettier's problem has attracted a range of more sophisticated responses. The different directions that these responses have taken are constrained by the structure of Gettier's argument: if knowledge is solely 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, in this account, one is to either accept Gettier's conclusion — and elucidate a new conceptual analysis for knowledge — or else deny one of Gettier's two claims about his counterexamples (that is, either deny that Gettier cases are justified true beliefs, or else accept that Gettier cases are knowledge after all).

One response, therefore, is that in none of the above cases was the belief justified: it is impossible to justify anything which is not true. Under this interpretation the JTB definition of knowledge survives. The problem is now not to define knowledge but to define justification.

However, most contemporary epistemologists accept Gettier's conclusion. Their responses to the Gettier problem, therefore, consist of trying to find alternative analyses of knowledge. They have struggled to discover and agree upon as a beginning any single notion of truth, or belief, or justifying which is wholly and obviously accepted. Truth, belief, and justifying have not yet been satisfactorily defined, so that JTB (justified true belief) may be defined satisfactorily is still problematical, on account or otherwise of Gettier's examples. Gettier, for many years a professor at University of Massachusetts Amherst later also was interested in the epistemic logic of Hintikka, a Finnish philosopher at Boston University, who published Knowledge and Belief in 1962. [1]

Fourth condition (JTB+G) approaches[edit]


Goldman's causal theory[edit]


Lehrer-Paxson's defeasibility condition[edit]

Keith Lehrer and Thomas Paxson (1969) proposed another 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 that, 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 Swain [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 a priori cases of knowledge[citation needed].


Pragmatism was developed as a philosophical doctrine by C.S.Peirce and William James (1842–1910). In Peirce's view, truth is nominally defined as a sign's correspondence to its object, and pragmatically defined as the ideal final opinion to which sufficient investigation would lead sooner or later. James' epistemological model of truth was that which works in the way of belief, and a belief was true if in the long run it worked for all of us, and guided us expeditiously through our semihospitable world. Peirce argued that metaphysics could be cleaned up by a pragmatic approach.


From a pragmatic viewpoint of the kind often ascribed to James, defining on a particular occasion whether a particular belief can rightly be said to be both true and justified is seen as no more than an exercise in pedantry, but being able to discern whether that belief led to fruitful outcomes is a fruitful enterprise. Peirce emphasized fallibilism, considered the assertion of absolute certainty a barrier to inquiry,[9] and in 1901 defined truth as follows: "Truth is that concordance of an abstract statement with the ideal limit towards which endless investigation would tend to bring scientific belief, which concordance the abstract statement may possess by virtue of the confession of its inaccuracy and one-sidedness, and this confession is an essential ingredient of truth.".[10] In other words, any unqualified assertion is likely to be at least a little wrong or, if right, still right for not entirely the right reasons; so one is more veracious by being Socratic and including a recognition of one's own ignorance, though in practical matters one sometimes must act, if one is to act at all, with decision and complete confidence of supposed knowledge even though one may end up proven wrong.[11]

Revisions of JTB approaches[edit]



Fred Dretske (1971) developed an account of knowledge which he called "conclusive reasons", revived by Robert Nozick as what he called the subjunctive or truth-tracking account (1981). Nozick's formulation posits that proposition P is an instance of knowledge when:

  1. p is true
  2. S believes that p
  3. if p were true, S would believe that p
  4. if p weren't true, S wouldn't believe that p



  1. p is true
  2. S believes that p



  • I see a barn


An alternate example is if Jones looks up and forms the belief

  • I see a red barn.


Richard Kirkham's skepticism[edit]


Attempts to dissolve the problem[edit]


  2. Gettier's cases really are cases of knowledge after all,


Those who accept (2) are by far in the minority in analytic 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.

Experimental research[edit]

Cross-cultural studies[edit]

Some recent work in the field of experimental philosophy has suggested that traditional intuitions about Gettier cases may actually vary cross-culturally. Jonathan Weinberg, Shaun Nichols and Stephen Stich conducted cross-cultural empirical studies in which participants were given examples of Gettier cases like the following:

Bob has a friend, Jill, who has driven a Buick for many years. Bob therefore thinks that Jill drives an American car. He is not aware, however, that her Buick has recently been stolen, and he is also not aware that Jill has replaced it with a Pontiac, which is a different kind of American car.

Participants were then asked:

Does Bob really know that Jill drives an American car, or does he only believe it?

While Western participants' responses were exactly what would have been expected by reading the philosophical literature, (Bob only believes that Jill drives an American car), the majority of East Asian participants actually reported the opposite (Bob truly knows that Jill drives an American car). A subsequent study was conducted with participants from the Indian subcontinent, and even more strongly divergent intuitions about Gettier cases were found.[12] There are many possible explanations for these findings and they may originate in translation ambiguities over the words "justified", "true", "belief" and "knowledge" and possibly depend on the mother-tongue make-up of the survey respondents.

However, subsequent studies were unable to replicate these results for other Gettier cases.[13] One possible explanation of this fact is that East Asian subjects were less familiar with American car models than their Western counterparts.


  1. ^ a b Timothy McGrew (2007), Internalism and Externalism, Abingdon, Oxon: Routledge , chapter 1
  2. ^ in 'Conditions of Knowledge' (1965). Chicago: Scott, Foresman
  3. ^ John L. Pollock; Joseph Cruz (1999), Contemporary theories of knowledge, Lanham, Md: Rowman & Littlefield Publishers, pp. 13–14, ISBN 0-8476-8936-0, 0847689360 
  4. ^ a b Alvin Plantinga (1992). Warrant: The Current Debate. Oxford University Press. pp. 6–7. ISBN 0-19-507862-4. 
  5. ^ Brian Skyrms "The Explication of `X knows that p'." Journal of Philosophy (June 22, 1967
  6. ^ Goldman, “Discrimination and Perceptual Knowledge”, The Journal of Philosophy, 1976
  7. ^ James Pryor. Theory of Knowledge - The Gettier Problem (archive)
  12. ^ Weinberg, J., Nichols, S. and Stich, S. 2001. “Normativity and Epistemic Intuitions,” Philosophical Topics, 29, 429–460.
  13. ^ Nagel, J. (forthcoming). "Intuitions and Experiments: A Defense of the Case Method in Epistemology". Philosophy and Phenomenological Research. 


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