->'''Beatrice:''' From now on, '''everything I speak in [[DontClickHere red]] is the truth'''. There is no need to doubt or question it.\\
'''Battler:''' And I should believe you when you say that? [[note]]It's later proved that, indeed, you cannot make a false statement in red.[[/note]]
-->-- The introduction of the Red Truth in ''VisualNovel/UminekoWhenTheyCry''.

A Language of Truth is a particular language or form of communication in which lies are inherently impossible or inexpressible. This is a frequent feature of {{Telepathy}},[[note]]because, as [[HighMiddleAges scholastics]] said, lying is saying one thing and thinking another, so you can't lie if you think and speak in the same act[[/note]] but it can also appear for more overt languages. A LanguageOfMagic may be this or, slightly differently, it may ''cause'' anything said in it to ''become'' true.

A RealLife LanguageOfTruth [[http://www.crockford.com/wrrrld/wilkins.html can't]] [[https://secure.wikimedia.org/wikipedia/en/wiki/Russell%27s_paradox possibly]] [[http://en.wikipedia.org/wiki/Gödels_incompleteness_theorem exist]] - not if you want it to be able to express ''every'' true statement. Consider:

-->This statement cannot be translated into a LanguageOfTruth.

Is this statement true, or is it false?
%% The primary focus of a trope article is the fictional trope -- short trope articles don't
%% benefit from big chunks of Real Life refutation.

Compare CannotTellALie, in which the inability to lie is inherent in individual characters.



[[folder: Comic Books ]]

* Telepathy functions this way in ''ComicBook/ElfQuest''. Unfortunately for them, the BigBad's mind is so malicious that it's ltierally painful to receive her thoughts. So most elves use vocal speech with her instead, and she can lie just fine with words.


[[folder: Fan Works ]]
* In ''FanFic/HarryPotterAndTheMethodsOfRationality'', Parseltongue, the languages of snakes is revealed to have this effect as neither Harry, Professor Quirrel or Voldemort were able to express direct deception through that language. It becomes an often demanded method of communication between the parties capable of understanding it to avoid such deception.

[[folder: Literature ]]

* The Old Speech in the ''Literature/{{Earthsea}}'' books, except to some extent for dragons:
-->While the Old Speech binds a man to truth, this is not so with dragons. It is their own language, and they can lie in it, twisting the true words to false ends, catching the unwary hearer in a maze of mirrorwords each of which reflects the truth and none of which leads anywhere.
* ''Literature/InheritanceCycle'': despite stating that elves are fond of half-truths and misdirection in the ancient language, it's never much of an issue. Although it is impossible to lie directly, lies can be told by thinking to yourself that you mean something else to what you are saying. [[spoiler:A lie can also be told if the speaker thinks it is true. (Although at that point it's more "being mistaken" than "lying", really.)]] Which partially explains how, in ''Eldest'', Eragon is able to compose a completely fictional story in verse that tells of a battlefield romance -- because he was thinking of Arya and it ''felt'' true. It's similar to what happens when Eragon tries to demonstrate a spell to take a sort of magic photograph by using it on Arya. It was supposed to be a perfect photo, and it's supposed to be really hard to fool the spell, but it... came out different, because of the way Eragon felt about Arya.
* Telepathy works this way in Creator/UrsulaKLeGuin's Literature/{{Hainish}} novels. The only exception is an alien race named Shing (the aliens in ''Literature/CityOfIllusions''). Apparently, they used that ability to overthrow TheFederation and take over... until a thousand or so years later, they were defeated by a race which was capable of detecting their lies.
* ''Literature/EarthsChildren'': The language of the Clan is like this. It's a sign language which supposedly involved so much careful observation of body language and expressions that any signs of lying would be obvious. Clan members can in fact lie, but can't get away with it, so nobody ever bothers -- to the point where the main character Ayla is confused by lying the first time she encounters it.
* ''Literature/WestOfEden'': The Yilanè in Creator/HarryHarrison's trilogy have this. Learning to speak is what makes a Yilanè yilanè (capable of speech), and the thought is what causes the body motions that make up their language unless they are completely still. Humans who learn the language, however, are capable of lying in it.
* ''Literature/NineteenEightyFour'': Newspeak was a decidedly dark take on this, in that the goal was to make unsayable anything not deemed "truth" by The Party/Big Brother.
* Obviously inspired by Orwell, the ''Literature/BookOfTheNewSun'' series features the Ascians, a people whose language is composed of a bunch of Mao-esque revolutionary slogans. However, the book explicitly shows that Ascians are capable of uttering subversive statements even when bound by this language.
* ''Literature/{{Hellspark}}'' by Creator/JanetKagan: It is technically possible to lie in Jenji, but the language is structured to provide as much accuracy and detail as possible, and is backed up by cultural and religious penalties for lying. Several times characters refer to speaking in Jenji as synonymous to telling the whole truth and nothing but the truth.
* In ''Literature/SectorGeneral'', the Kelgian speech and telepathic communication technically qualify. Kelgian languages are perfectly ordinary: the fun part is that their vocal speech is inextricably linked with the involuntary expression of emotions in the movements of their fur. The species developed no concept of lies. Or tact. They're not so hot on bedside manner, either. Telepathic contact is full awareness of the partner's psyche, making lies exceptionally pointless.
* In Neal Stephenson's ''[[Literature/TheBaroqueCycle Quicksilver]]'', philosophers are attempting to make a truly logical language which, when used correctly, makes it impossible to say things which are false. The idea is that it would work like algebra: valid algebraic transformations on correct equations always lead to other correct equations. This is a little bit different from some of the other examples, since it protects not only against the speaker lying, but also against the speaker mistakenly saying something wrong.
* In Creator/EEDocSmith's ''Literature/{{Lensman}}'' series, it is absolutely impossible to make an untrue state telepathically, and the recipient knows this, with absolute certainty. It is possible, however, to make a correct but uninformative statement, such as "I cannot divulge that."
* ''Literature/HeraldsOfValdemar'': In Mercedes Lackey's Velgarth novels, only the ''dyheli,'' an artificial race of deer-like sapients with truly phenomenal telepathic abilities, can lie with Mindspeech.
* In the ''Literature/YoungWizards'' series, the Speech is the language that defines the fabric of the universe. Even the [[BigBad Lone Power]] can't lie in it. Although as the Speech can express every concept, it includes both words and grammar which are inaudible, incomprehensible, or guaranteed to be predictably misunderstood by any non-omniscient listener. Beings on the order of the Lone Power have a lot of potential wiggle room, though the book series only exploits this to the extent of allowing the Lone Power conversational freedom in the Speech. When used in spells as the LanguageOfMagic, saying or writing something in the Speech is what ''makes'' something true, which makes truthful descriptions of things especially important.
* Creator/NancyKress's ''Probability: Space'' series has a race that experiences a consensual, shared reality, and can't say things that disagree with the consensus. It's not so much that they can't lie--but they can't lie unless they all believe the lie.
* In Creator/ChinaMieville's ''Literature/{{Embassytown}}'', the Ariekei speak a very unusual language--in fact, the oddness of the language is a central part of the whole book--but one of its features is that they can't really lie. Some of them are trying to learn, however, by starting a statement, and then saying the second half too quietly to be heard.
* In Creator/DavidEddings' ''Literature/{{Belgariad}}'' and ''Literature/{{Malloreon}}'' series, it is impossible to tell a lie in the language of wolves. At one point a wolf wants Ce'nedra to stop talking. Garion tells her the wolf wants to sleep. When he tells the wolf to close her eyes to pretend to sleep, the wolf is amazed that is possible to say things that aren't true in the human language.
* In several of Creator/MarionZimmerBradley's works, being under the effect of a truthspell makes any language work like this. However, while you can't ''lie'', you certainly can say things that are not true provided that you believe them to be true. It's also specifically noted that in situations where an objective "truth" is ambiguous or nonexistent (for example, most political disputes), truthspells are next to useless.

[[folder: Live-Action TV]]
* ''Franchise/StarTrek'':
** This is played with and ultimately subverted in ''Series/StarTrekDeepSpaceNine''. For a long time, Odo [[BlatantLies insists]] that [[MentalFusion linking]] with other [[VoluntaryShapeshifter Changelings]] "isn't about information exchange." Then they accidentally reveal the identity of a Changeling infiltrator to him, and the entire cast assumes this trope is in effect. [[spoiler: It isn't. The Changelings deliberately misled Odo.]]
** Played straight with most other forms of telepathy in the Star Trek universe.

[[folder: Tabletop Games ]]

* ''TabletopGame/InNomine'' has the divine language, which is impossible to lie in. The demons don't like that, so after the initial fall they create a butchered version, called Dark Celestial.


[[folder: Visual Novels ]]
* ''VisualNovel/UminekoWhenTheyCry'' has the red truth. If anything is said in red text, it must be true. If it's not true, it cannot be said in red text. As well as being vitally important to solving the mystery of the island, it gives us such gems as '''You are incompetent!''' and '''"kukkukukukukuku... hihihihihihihihihihihyahyahyahhahhahahaha-hhahahahhahhahhahhahhahhahha!"''' In addition, if someone attempts to lie using the red text, [[spoiler:they will choke on the words that they're trying to say.]] It should be noted though, that the red expresses something that is true ''from the point view of the speaker''. Hence why Beatrice can say such things as "You are incompetent!" or [[spoiler:"Kanon was killed in this room."]]

[[folder: Webcomics ]]

* ''Webcomic/GeneCatlow'': The "Sight of the Soul" telepathic communication works like this - since it's direct communication between souls, it doesn't allow for lies, falsehood, or even withholding the truth. However, recent comics have revealed that, at least at one point, there existed a type of etherical being with a mind so insane and fractured that it was capable of lying through the Sight of the Soul...
* ''Webcomic/TalesOfTheQuestor'' recommends that if you're going to [[DealWithTheDevil cut a deal with ]][[TheFairFolk a fae lord,]] Latin is the language to do it in. Being a dead language, [[LiteralGenie meanings]] are fixed and specific. There's also a brief mention of floriography, or [[http://en.wikipedia.org/wiki/Floriography the Language of the Flowers,]] which allegedly carries similar weight.
* Averted in ''Webcomic/ErrantStory'', where a telepathic [[http://www.errantstory.com/2004-02-23/202 fib]] by Sarine is crucial to seven more years of plot development.
* In ''Webcomic/{{Roommates}}'' things said in double lined letters (the Voice of Truth & Magic) are true and not even subjectively true as it even makes mistakes and the like impossible. The only one who ever uses it is Jareth the fae roommate, mostly to magically bind himself to his given word. Considering that nobody tends to trust him and he has a SuperpoweredEvilSide this is a wise precaution. The one time when he was trying but not able to promise something in it that was actually a plot point.

[[folder: Western Animation ]]

* ''WesternAnimation/TheFairlyOddparents'' : Inverted; Timmy wishes to "always be right". Rather than making him only say correct statements, any claim he makes [[RealityWarper instantly becomes true]].