Liar paradox

Liar paradox

In philosophy and logic, the liar paradox, known to the ancients as the pseudomenon, encompasses paradoxical statements such as "This sentence is false." or "The next sentence is false. The previous sentence is true." These statements are paradoxical because there is no way to assign them a consistent truth value. If "This statement is false" is true, then what it says is the case; but what it says is that it is false, hence it is false. On the other hand, if it is false, then what it says is not the case; thus, since it says that it is false, it must be true.


In the sixth century BC the philosopher-poet Epimenides, himself a Cretan, reportedly wrote:

:"The Cretans are always liars."

The Epimenides paradox is often considered equivalent or interchangeable with the "liar paradox", but they are not the same. The liar paradox is a statement that cannot consistently be true or false, while Epimenides' statement is simply false, as long as there exists at least one Cretan who sometimes tells the truth.

It is unlikely that Epimenides intended his words to be understood as a kind of liar paradox, and they were probably only understood as such much later in history. The oldest known version of the liar paradox is instead attributed to the Greek philosopher Eubulides of Miletus who lived in the fourth century BC. It is very unlikely that he knew of Epimenides's words, even if they were intended as a paradox. Eubulides reportedly said::"A man says that he is lying. Is what he says true or false?"

Explanation of the paradox and variants

The problem of the liar paradox is that it seems to show that common beliefs about truth and falsity actually lead to a contradiction. Sentences can be constructed that cannot consistently be assigned a truth value even though they are completely in accord with grammar and semantic rules.

The simplest version of the paradox is the sentence:

:"This statement is false". (A)

If the statement is true, everything asserted in it must be true. However, because the statement asserts that it is itself false, it must be false. So the hypothesis that it is true leads to the contradiction that it is false. Yet the sentence cannot be false for that hypothesis also leads to contradiction. If the statement is false, then what it says about itself is not true. It says that it is false, so that must not be true. Hence, it is true. Under either hypothesis, the statement is both true and false.

However, that the liar sentence can be shown to be true if it is false and false if it is true has led some to conclude that it is "neither true nor false". This response to the paradox is, in effect, to reject the common beliefs about truth and falsity: the claim that every statement has to abide by the principle of bivalence, a concept related to the law of the excluded middle.

The proposal that the statement is neither true nor false has given rise to the following, strengthened version of the paradox:

:"This statement is not true." (B)

If (B) is neither true nor false, then it must be not true. Since this is what (B) itself states, it means that (B) must be true and so one is led to another paradox.

Another reaction to the paradox of (A) is to posit, as Graham Priest has, that the statement follows paraconsistent logic and is "both true and false". Nevertheless, even Priest's analysis is susceptible to the following version of the liar:

:"This statement is only false." (C)

If (C) is both true and false then it must be true. This means that (C) is only false, since that is what it says, but then it cannot be true, creating another paradox.


The statement "I always lie" is often considered to be a version of the liar paradox, but is not actually paradoxical. It could be the case that the statement itself is a lie, because the speaker sometimes tells the truth, and this interpretation does not lead to a contradiction. The belief that this is a paradox results from a false dichotomy - that either the speaker always lies, or always tells the truth - when it is possible that the speaker occasionally does both.

Possible resolutions

Alfred Tarski

Alfred Tarski diagnosed the paradox as arising only in languages that are "semantically closed" by which he meant a language in which it is possible for one sentence to predicate truth (or falsehood) of another sentence in the same language (or even of itself). To avoid self-contradiction, it is necessary when discussing truth values to envision levels of languages, each of which can predicate truth (or falsehood) only of languages at a lower level. So, when one sentence refers to the truth-value of another, it is semantically higher. The sentence referred to is part of the "object language," while the referring sentence is considered to be a part of a "meta-language" with respect to the object language. It is legitimate for sentences in "languages" higher on the semantic hierarchy to refer to sentences lower in the "language" hierarchy, but not the other way around. This prevents a system from becoming self-referential.

A. N. Prior

A. N. Prior asserts that there is nothing paradoxical about the liar paradox. His claim (which he attributes to Charles S. Peirce and John Buridan) is that every statement includes an implicit assertion of its own truth. Thus, for example, the statement "It is true that two plus two equals four" contains no more information than the statement "two plus two equals four," because the phrase "it is true that..." is always implicitly there. And in the self-referential spirit of the Liar Paradox, the phrase "it is true that..." is equivalent to "this whole statement is true and ...".

Thus the following two statements are equivalent:

:"This statement is false":"This statement is true and this statement is false."

The latter is a simple contradiction of the form "A and not A", and hence is false. There is therefore no paradox because the claim that this two-conjunct Liar is false does not lead to a contradiction. Eugene Mills [Mills, Eugene (1998) ‘A simple solution to the Liar’, Philosophical Studies 89: 197-212.] and Neil Lefebvre and Melissa Schelein [Lefebvre, N. and Schelein, M., "The Liar Lied," in Philosophy Now issue 51] present similar answers.

aul Kripke

Saul Kripke points out that whether a sentence is paradoxical or not can depend upon contingent facts. If the only thing Smith says about Jones is

:"A majority of what Jones says about me is false."

and Jones says only these three things about Smith:

:"Smith is a big spender." :"Smith is soft on crime.":"Everything Smith says about me is true."

and Smith really is a big spender but is "not" soft on crime, then both Smith's remark about Jones and Jones's last remark about Smith are paradoxical.

Kripke proposes a solution in the following manner. If a statement's truth value is ultimately tied up in some evaluable fact about the world, that statement is "grounded". If not, that statement is "ungrounded". Ungrounded statements do not have a truth value. Liar statements and liar-like statements are ungrounded, and therefore have no truth value.

Barwise and Etchemendy

Jon Barwise and John Etchemendy propose that the liar sentence (which they interpret as synonymous with the Strengthened Liar) is ambiguous. They base this conclusion on a distinction they make between a "denial" and a "negation". If the liar means "It is not the case that this statement is true" then it is denying itself. If it means "This statement is not true" then it is negating itself. They go on to argue, based on their theory of "situational semantics", that the "denial liar" can be true without contradiction while the "negation liar" can be false without contradiction.


Graham Priest and other logicians have proposed that the liar sentence should be considered to be both true "and" false, a point of view known as dialetheism. In a dialetheic logic, all statements must be either true, or false, or both. Dialetheism raises its own problems. Chief among these is that since dialetheism recognizes the liar paradox, an intrinsic contradiction, as being true, it must discard the long-recognized principle of "ex falso quodlibet", which asserts that any sentence whatsoever can be deduced from a true contradiction. Thus, dialetheism only makes sense in systems that reject "ex falso quodlibet". Such logics are called "paraconsistent".

Logical structure of the liar paradox

For a better understanding of the liar paradox, it is useful to write it down in a more formal way. If "this statement is false" is denoted by A and its truth value is being sought, it is necessary to find a condition that restricts the choice of possible truth values of A. Because A is self-referential it is possible to give the condition by an equation.

If some statement, B, is assumed to be false, one writes "B = false". The statement (C) that the statement B is false would be written as "C = "B = false". Now, the liar paradox can be expressed as the statement A, that A is false:

"A = "A = false"

This is an equation from which the truth value of A = "this statement is false" could hopefully be obtained. In the boolean domain "A = false" is equivalent to "not A" and therefore the equation is not solvable. This is the motivation for reinterpretation of A. The simplest logical approach to make the equation solvable is the dialetheistic approach, in which case the solution is a A being both "true" and "false". Other resolutions mostly include some modifications of the equation e.g. A. N. Prior claims that the equation should be "A = "A = false" and "A = true" and therefore A is false.

ee also

*Quine's paradox
*List of paradoxes



* Jon Barwise and John Etchemendy (1987) "The Liar". Oxford University Press.
* Greenough, P.M., (2001) " ," "American Philosophical Quarterly 38":
* Hughes, G.E., (1992) "John Buridan on Self-Reference : Chapter Eight of Buridan's Sophismata, with a Translation, and Introduction, and a Philosophical Commentary", Cambridge Univ. Press, ISBN 0-521-28864-9. Buridan's detailed solution to a number of such paradoxes.
* Kirkham, Richard (1992) "Theories of Truth". MIT Press. Especially chapter 9.
* Saul Kripke (1975) "An Outline of a Theory of Truth," "Journal of Philosophy 72": 690-716.
* Lefebvre, Neil, and Schelein, Melissa (2005) "The Liar Lied," "Philosophy Now" issue 51.
* Graham Priest (1984) "The Logic of Paradox Revisited," "Journal of Philosophical Logic 13": 153-179.
* A. N. Prior (1976) "Papers in Logic and Ethics". Duckworth.
* Smullyan, Raymond (19nn) "What is the Name of this Book?". ISBN 0-671-62832-1. A collection of logic puzzles exploring this theme.

External links

* Internet Encyclopedia of Philosophy: " [ Liar Paradox] " -- by Bradley Dowden.

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