fal·​la·​cy | \ˈfa-lə-sē \
plural fallacies

Definition of fallacy 

1a : a false or mistaken idea popular fallacies prone to perpetrate the fallacy of equating threat with capability— C. S. Gray

b : erroneous character : erroneousness The fallacy of their ideas about medicine soon became apparent.

2a : deceptive appearance : deception

b obsolete : guile, trickery

3 : an often plausible argument using false or invalid inference

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Did You Know?

Philosophers are constantly using the word fallacy. For them, a fallacy is reasoning that comes to a conclusion without the evidence to support it. This may have to do with pure logic, with the assumptions that the argument is based on, or with the way words are used, especially if they don't keep exactly the same meaning throughout the argument. There are many classic fallacies that occur again and again through the centuries and everywhere in the world. You may have heard of such fallacies as the "ad hominem" fallacy, the "question-begging" fallacy, the "straw man" fallacy, the "slippery slope" fallacy, the "gambler's" fallacy, or the "red herring" fallacy. Look them up and see if you've ever been guilty of any of them.

Examples of fallacy in a Sentence

The fallacy of their ideas about medicine soon became apparent. the once-common fallacy that girls just weren't any good at math

Recent Examples on the Web

Sign up for our daily newsletter and never miss our latest and greatest stories Culture A guide to busting through confirmation bias, the cognitive fallacy that's destroying our discourse. Michael Fitzgerald, WIRED, "The Court Case that Enabled Today's Toxic Internet," 8 July 2018 And there is another essential fallacy of these negotiations. NBC News, "North Korea shatters Trump's boastful assurances of an easy path to denuclearization," 7 July 2018 Technologists have lauded automated decision-making as a way to further reduce human fallacy. Joshua Brustein, Bloomberg.com, "Impress the Algorithm. Get $250,000," 1 May 2018 Even Doocy, an ardent Trump apologist, seemed flabbergasted by the logical fallacy of Trump's deep-state conspiracy theory. Callum Borchers, Washington Post, "‘It’s your Justice Department!’: Fox News hosts tried and failed to halt Trump’s rant," 26 Apr. 2018 The fallacies in Korematsu were echoed in the travel ban ruling, warned Hiroshi Motomura, a University of California, Los Angeles, law professor who has written extensively about immigration. Charlie Savage, New York Times, "Korematsu, Notorious Supreme Court Ruling on Japanese Internment, Is Finally Tossed Out," 26 June 2018 The Facts One fallacy that non-journalists often make is that reporters are handed leaks on a silver patter. Glenn Kessler, Washington Post, "The unsupported claim that James Clapper tipped Jake Tapper about the dossier," 3 May 2018 But this is just another fallacy in the administration’s official reasoning. Eric Levitz, Daily Intelligencer, "Trump Could Enforce the Border Without Locking Up Families," 25 June 2018 The popular fallacy now is the Reds are winning because their starting pitching is dealing. Paul Daugherty, Cincinnati.com, "Daugherty: Enjoy the impossible while it lasts, Cincinnati Reds fans," 24 June 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'fallacy.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of fallacy

15th century, in the meaning defined at sense 2b

History and Etymology for fallacy

Latin fallacia, from fallac-, fallax deceitful, from fallere to deceive

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Statistics for fallacy

Last Updated

25 Oct 2018

Look-up Popularity

Time Traveler for fallacy

The first known use of fallacy was in the 15th century

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More Definitions for fallacy



English Language Learners Definition of fallacy

: a wrong belief : a false or mistaken idea

: the quality of being false or wrong


fal·​la·​cy | \ˈfa-lə-sē \
plural fallacies

Kids Definition of fallacy

1 : a false or mistaken idea

2 : false reasoning

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