Battler: And I should believe you when you say that?
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 but it can also appear for more overt languages. A Language of Magic may be this or, slightly differently, it may cause anything said in it to become true - this is known in Real Life as "magical notion of truth" . Beware, of course, the Consummate Liar who is either outright immune or uses Exact Words and other forms of deception to get around this inconvenience.
- Telepathy functions this way in ElfQuest. Unfortunately for them, the Big Bad's mind is so malicious that it's literally painful to receive her thoughts. So most elves use vocal speech with her instead, and she can lie just fine with words.
- In The Chronicles of Wormwood, it is impossible to say anything untrue in the presence of Jay, who happens to be the Second Coming of Jesus Christ. It doesn't matter if you're actually trying to deceive him (if, for example, you're testing the effect with a ridiculous lie like "My parents were secretly elephants"), you just can't finish the sentence. People who are trying to deceive him, even for polite reasons, have a tendency to say what they really mean instead.
- In Harry Potter and the Methods of Rationality, Parseltongue, the language of snakes, is revealed to have this effect as neither Harry, Professor Quirrell 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. This does not, however, prevent them from telling misleading truths.
- The Old Speech in the 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.
- The implication of the Earthsea Old Speech is that what is said in it, if it is not already true, becomes true, even if the world has to bend to make it so. The dragons are simply old enough and crafty enough to say false things in ways that won't change reality.
- Inheritance Cycle: 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. The language also doesn't contain any measures to stop a speaker from being mistaken. 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 Ursula K. Le Guin's Hainish novels. The only exception is an alien race named Shing (the aliens in City of Illusions). Apparently, they used that ability to overthrow The Federation and take over... until a thousand or so years later, they were defeated by a race which was capable of detecting their lies.
- Earth's Children: 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.
- Before they became Living Lie Detectors for law enforcement, the ghatti of the Ghatti's Tale series first had to learn what lying was, as like the Clan, their language is a gestural one in which successful lying is impossible.
- West of Eden: The Yilanè in Harry Harrison'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.
- 1984: 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 Book of the New Sun 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.
- Hellspark by Janet Kagan: 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 Sector General, 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 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 E. E. "Doc" Smith's 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."
- Heralds of Valdemar: 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 Young Wizards series, the Speech is the language that defines the fabric of the universe. Even the 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 Language of Magic, saying or writing something in the Speech is what makes something true, which makes truthful descriptions of things especially important.
- Nancy Kress'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 China Miéville's 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 David Eddings' Belgariad and 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. (Interestingly, while the wolf's dialogue wasn't false, it ultimately turns out to been an expression of deception regardless.)
- In several of Marion Zimmer Bradley'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.
- In Robert A. Heinlein's Between Planets the language of the Venerian "dragons" is called "true speech" and as one (human) character points out, doesn't even have a word for the concept of deliberate falsehood, meaning that Venerians (at least when they're speaking their own language) are unable to lie.
- Star Trek:
- This is played with and ultimately subverted in Star Trek: Deep Space Nine. For a long time, Odo insists that linking with other 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. It isn't. The Changelings deliberately misled Odo.
- Played straight with most other forms of telepathy in the Star Trek universe.
- The Twilight Zone (1959) features an example in the episode "Hocus-Pocus and Frisby." The eponymous Frisby constantly tells Blatant Lies about his accomplishments. Such lies range from the acquisition of degrees, attending prominent schools and being able to solve all sorts of mathematical equations. The town is privy to the fact that Frisby is a liar and chide him often about it. Others just marvel that he doesn't strive for any sort of consistency and point out that thus far he'd have to have been alive during three different eras for his tales to be true. An exception comes in the form of a pair of aliens. They believe every one of Frisby's lies. Realizing that he is about to be abducted, he confesses that he's little more than a big mouthed liar. The aliens explain to him that they don't understand the word "lie" and that there isn't a single equivalent in their native language that comes close to describing what that is.
- In Nomine 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.
- Numenera has Truth, a scholarly language which the Church says cannot be used to lie. Played with, however, in that not everyone buys this.
- Umineko: When They Cry:
- Its most famous example is 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. An infallible language of truth sounds like an incredible power in a Mystery Fiction — but it's mostly used by supernatural beings to torment the protagonist with information that doesn't need to be evidenced or explained. In particular, while red truth can't be used to outright lie, it can (and will) be definitely used to mislead and deceive. One major reveal comes when a character tries to say something in red text and fails, much to his own surprise.
- Less famous is the gold truth. It seems to work exactly the same as the red truth, except with the additional requirement that the speaker "understand the rules".
- Zigzagged by the purple truth: While most people cannot lie in purple, those who have committed murder can.
- Gene Catlow: 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, 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...
- Tales of the Questor recommends that if you're going to cut a deal with a fae lord, Latin is the language to do it in. Being a dead language, meanings are fixed and specific. There's also a brief mention of floriography, or the Language of the Flowers, which allegedly carries similar weight.
- In 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 Superpowered Evil Side 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.