fallacy

noun

fal·​la·​cy ˈfa-lə-sē 

plural fallacies

1

a

: 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.

2

a

: deceptive appearance : deception

b

obsolete : guile, trickery

3

: an often plausible argument using false or invalid inference

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.

Synonyms

• delusion
• error
• falsehood
• falsity
• hallucination
• illusion
• misbelief
• misconception
• myth
• old wives' tale
• untruth

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

Landman, set in the oil fields of West Texas, is punctuated by extensive diatribes from Tommy about the oil industry’s importance, the tragic necessity of landmen and roughnecks, and the fallacy of placing society’s hope on renewable energy. —Nicholas Quah, Vulture, 20 Dec. 2024

The argument that somehow voters in more populous states are disenfranchised is also a fallacy. —Joe Battenfeld, Boston Herald, 19 Dec. 2024

Already a #1 Sunday Times bestseller and has been shortlisted for the Nero Book Awards, An African History of Africa corrects many of the misguided fallacies that pervade Western narratives about Africa. —Lynnette Nicholas, Essence, 18 Dec. 2024

The report focuses on the fallacies in these arguments. —Marybeth Gasman, Forbes, 15 Dec. 2024

Word History

Etymology

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

First Known Use

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

Phrases Containing fallacy

• pathetic fallacy

Kids Definition

fallacy

noun

fal·​la·​cy ˈfal-ə-sē 

plural fallacies

1

: a false or mistaken idea

2

: the quality or state of being false