fal·​la·​cy | \ ˈfa-lə-sē How to pronounce fallacy (audio) \
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

The project explained basic statistics, as well as concepts such as how to evaluate source quality, logical fallacy, emotionally evocative language. Gwen Moran, Fortune, "We’re in a Data Literacy Crisis. Could Librarians Be the Superheroes We Need?," 31 Aug. 2019 Prosecution descriptions of an empire that paid for private planes, beachfront villas and a private zoo were a fallacy, his lawyers say. Tom Hays, Anchorage Daily News, "Drug lord Joaquín ‘El Chapo’ Guzmán gets life sentence in U.S. federal court," 17 July 2019 Yet this hides the second fallacy: the negative association can be affected by many other factors. Alberto Cairo, Scientific American, "Graphics That Seem Clear Can Easily Be Misread," 21 Aug. 2019 This was once a very specific term referring to any of several very specific logical fallacies. June Casagrande, Burbank Leader, "A Word, Please: Grammar blunders that have crept into regular English usage," 15 Aug. 2019 But look closely, and see the fallacy of the American Dream by way of immigration issues. Darcel Rockett, chicagotribune.com, "29Rooms pop-up is back in Chicago, promising to ‘expand your reality.’ Here’s a look inside.," 18 July 2019 People being offended about this whole Jackie O thing should really look into the whole fallacy of the Kennedy’s and Camelot. Maria Puente, USA TODAY, "President Trump likens first lady Melania Trump to 'Jackie O' but Twitter objects," 14 June 2019 The reasoning is completely sound, except for the implicit assumption that a finite object can be placed at the focus, which is the source of the fallacy. Quanta Magazine, "Solution: ‘Is Infinity Real?’," 30 June 2016 The flawed interpretations of the original paper fell afoul of what statisticians call the ecological fallacy, when an inference is made about an individual based on statistical data for a group. The Economist, "Medical marijuana is not the way out of America’s opioid crisis," 24 June 2019

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

13 Sep 2019

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 How to pronounce fallacy (audio) \
plural fallacies

Kids Definition of fallacy

1 : a false or mistaken idea
2 : false reasoning

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Comments on fallacy

What made you want to look up fallacy? Please tell us where you read or heard it (including the quote, if possible).


recurring in steady succession

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