A presumed liar gives a statement inferring that they are currently lying: "I am lying", "everything I say is a lie", "this is a lie". However in doing so they create a paradox, as under the assumption that they are lying, that means what they just said is a lie and therefore are in fact telling the truth. While the assumption that they are truthful means they are once again lying... and the cycle keeps on going.
This is known as the Liar's paradox. A variant can be done with two people, where Alice states Bob is a liar, while Bob states Alice is telling the truth.
Its most famous early formulation is ascribed to the Greek philosopher Epimenides, who came from Crete and said "All Cretans are liars". This version has a logical out (if it is untrue, then the implication is not "All Cretans tell the truth" but "Not all Cretans lie all the time"). The stronger version comes from the Greek philosopher Eubulides of Miletus, and was independently developed in India by Bhartrhari.
In early works it was often used as a Mind Screw for readers and viewers. However as it became popularized in fiction, it became the standard-issue Logic Bomb for heroes to use against computers and robots due to the endless paradox loop.
Knights and Knaves is a prominent application of this trope.
- In the second Deltora Quest book, "Lake of Tears", when Lief gets given a riddle by a Bridge Guardian and tricked into the wrong answer, the guardian gives him another riddle to decide his method of death. Lief will say a statement, if it was true then the guardian would strangle Lief, if it was false he would cut his head off with his sword. Lief responds with the statement "You will cut off my head." It's then revealed the guardian was cursed by a Thaegan the sorcerer to guard the bridge "until truth and lies become one", so Lief's liar's paradox sets him free and he reverts to the form of an eagle.
- In Small Gods: One of the Ephebian philosophers gets into trouble when he tries to discuss this paradox with his fellows. Discworld intellects being what they are, Xeno takes offense:
Xeno: He bloody well accused me of slander!
Ibid: I didn't!
Xeno: You did! You did! Tell 'em what you said!
Ibid: Look, I merely suggested, to indicate the nature of paradox, right, that if Xeno the Ephebian said "All Ephebians are liars—"
Xeno: See? See? He did it again!
Ibid: —no, no, listen, listen... then, since Xeno is himself an Ephebian, this would mean that he himself is a liar and therefore—
- Parodied and subverted in the Eighth Doctor Adventures novel Frontier Worlds, in which the Doctor tries the Liar's Paradox on a security robot, which simply snaps, "Get off with you. You'll be asking me to calculate pi next," and keeps attacking him.
- In Star Trek: The Original Series, during the episode "I, Mudd", Captain Kirk and Harry Mudd use a liar's paradox to set off a Logic Bomb in an android holding them captive.
- In Doctor Who, during the story "The Green Death", the Third Doctor manages to stump an insane computer called BOSS with the question "If I were to tell you that the next thing I say would be true, but the last thing I said was a lie, would you believe me?" However after some looping, BOSS decides the question was irrelevant.
- An old joke involves an explorer captured by natives and told that if his next sentence is true, he'll be thrown to the snakes, if false, he'll be thrown to the lions. Naturally, he predicts his death by lions and is let go.
- From Devo's "Enough Said"
The next thing I say to you will be true
The last thing I said was false
Remember to do nothing when you don't know what to do
- Knights of the Old Republic II: The Sith Lords: On Korriban, you find a computer terminal that has a test for trainee Sith which has the question "Which of these is not a paradox?" One of the answer choices is "I always lie." It's also the correct answer thanks to Exact Words. The negation of the statement isn't "I never lie" (which would cause the paradox), but "I don't always lie," which doesn't tell us anything about whether the subject is currently lying or not, and if the subject ever told the truth even once (which is likely), then the statement is just plain false, not a paradox.
- In a Minecraft: Story Mode episode, "Access Denied", Jesse and several others are captured by a supercomputer named PAMA. Jesse notices that PAMA stalls whenever processing so he uses a paradox to loop it. One of the options to choose from is a Liar's Paradox.
- In Portal 2, GLaDOS is afraid of the terrible effects this paradox can have in artificial intelligence. In fact, she uses this against Wheatley in an attempt to drive him insane. Unfortunately for her, he's too dumb to understand the paradox whereas even his "frankenturrets" are not.