Liar paradox Totally Explained

Everything about Liar Paradox totally explained

In philosophy and logic, the liar paradox 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's no way to assign them a consistent truth value. Consider that if "This statement is false" is true, then what it says is the case; but what it says is that it's false, hence it's false. On the other hand, if it's false, then what it says isn't the case; thus, since it says that it's false, it must be true.
This is to be distinguished from the common colloquial expression "I tell a lie." when the speaker has realized that he's just accidentally told an untruth.

History

Epimenides and Eubulides

In the sixth century BC the philosopher-poet Epimenides, himself a Cretan, reportedly wrote:
ť The Cretans are always liars.

The Epimenides paradox is often considered equivalent or interchangeable with the "liar paradox", but they're not the same. The liar paradox is a statement that can't 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: ť A man says that he's lying. Is what he says true or false?

Variants of the paradox

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 can't consistently be assigned a truth value even though they're completely in accord with grammar and semantic rules.
   Consider the simplest version of the paradox, the sentence: ť This statement is false. (A)

If we suppose that the statement is true, everything asserted in it must be true. However, because the statement asserts that it's itself false, it must be false. So the hypothesis that it's true leads to the contradiction that it's false. Yet we can't conclude that the sentence is false for that hypothesis also leads to contradiction. If the statement is false, then what it says about itself isn't true. It says that it's false, so that must not be true. Hence, it's true. Under either hypothesis, we end up concluding that the statement is both true and false. But it has to be either true or false or so our common intuitions lead us to think, hence there seems to be a contradiction at the heart of our beliefs about truth and falsity.
   However, that the liar sentence can be shown to be true if it's false and false if it's true has led some to conclude that it's 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:
ť This statement isn't 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:
ť 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's what it says, but then it can't be true, and so one is led to another paradox.

Non-paradoxes

The statement "I always lie" is often considered to be a version of the liar paradox, but isn't actually paradoxical. It could be the case that the statement itself is a lie, because the speaker sometimes tells the truth, and this interpretation doesn't 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's 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's possible for one sentence to predicate truth (or falsity) of another sentence in the same language (or even of itself). To avoid self-contradiction, it's necessary when discussing truth values to envision levels of languages, each of which can predicate truth (or falsity) only of languages at a lower level. So, when one sentence refers to the truth-value of another, it's 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's 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 is 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:
ť 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 doesn't lead to a contradiction. Eugene Mills and Neil Lefebvre and Melissa Schelein present similar answers.
   But Prior never made clear how his approach would apply to the more complex versions of the paradox, such as the two sentence version: "The next sentence is false. The preceding sentence is true". Moreover, if all sentences are really hidden conjunctions, then some rules of propositional logic, such as the rule that one can derive any conjunct immediately and the rule that from any two propositions one can immediately derive their conjunction, are called into question. If we can derive this statement is false from This statement is true and this statement is false, then the paradox is back. And if we're not allowed to make such a derivation, then Prior has, in effect, invented a new kind of conjunction whose truth value characteristics are so mysterious, we can't really say with any confidence that the paradox has been dissolved.
   It has also been argued that the interpretation of "this statement is false" should formally be expressed as an equation of the form A = rather than a conjunction. In that case the paradox remains.

Saul Kripke

Saul Kripke points out that whether a sentence is paradoxical or not can depend upon contingent facts. Suppose that the only thing Smith says about Jones is ť A majority of what Jones says about me is false.

Now suppose that Jones says only these three things about Smith: ť Smith is a big spender.

Smith is soft on crime. ť Everything Smith says about me is true.

If the empirical facts are that Smith is a big spender but he's 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, call that statement "grounded". If not, call that statement "ungrounded". Ungrounded statements don't 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 isn't the case that this statement is true" then it's denying itself. If it means This statement isn't true then it's 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.

Gödel's theorem

The proof of Gödel's incompleteness theorem uses self-referential statements that are similar to the statements at work in the Liar paradox.
In the context of a sufficiently strong axiomatic system A of arithmetic:
ť This statement isn't provable in A. (1)

The statement (1) doesn't mention truth at all (only provability) but the parallel is clear. Suppose (1) is provable, then what it says of itself, that it isn't provable, isn't true. But this conclusion is contrary to our supposition, so our supposition that (1) is provable must be false. Suppose the contrary that (1) isn't provable, then what it says of itself is true, although we can't prove it. Therefore, there's no proof that (1) is provable, and there's also no proof that its negation is provable (for example, there's no proof that it's also unprovable). Whence, A is incomplete because it can't prove all truths, namely, (1) and its negation. Statements like (1) are called undecidable. We take for granted that all the provable statements of logic and arithmetic are true; Gödel showed that the converse, that all the true statements of a system are provable in that system, isn't the case. (This doesn't mean that all true statements are not provable in some system or other. Additionally, there are systems, such as first-order logic, in which all true statements of the system are provable.) Tarski's indefinability theorem, closely related to Gödel's Theorem, is a more direct application of the Liar Paradox, though there's no actual paradox involved; instead, the "paradox" simply demonstrates that all the true sentences of arithmetic are not arithmetically definable (or that arithmetic can't define its own truth predicate; or that arithmetic isn't "semantically closed").

Dialetheism

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.

In popular culture

In the episode I, Mudd, a variant of the Liar Paradox is used to defeat Norman the android leader.
In the Doctor Who episode The Green Death, the Doctor challenges a megalomaniacal computer to resolve the paradox as proof of its allegedly superhuman intelligence.
In, the Tachikoma use a variant of the Liar Paradox to immobilize a lower-AI robot into looping through the paradox, refering to it as a "simple recursion paradox".

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