In logic, equivocation ("calling two different things by the same name") is an informal fallacy resulting from the failure to define one's terms, or knowingly and deliberately using words in a different sense than the one the audience will understand.

It is a type of ambiguity that stems from a phrase having two or more distinct meanings, not from the grammar or structure of the sentence.

Fallacy of four terms

Equivocation in a syllogism (a chain of reasoning) produces a fallacy of four terms (). Below is an example:

Since only man [human] is rational. And no woman is a man [male]. Therefore, no woman is rational.

The first instance of "man" implies the entire human species, while the second implies just those who are male.

Motte-and-bailey fallacy

Equivocation can also be used to conflate two positions which share similarities, one modest and easy to defend and one much more controversial. The arguer advances the controversial position, but when challenged, they insist that they are only advancing the more modest position.

See also

Antanaclasis: A related purposeful rhetorical device Circumlocution: Phrasing to explain something without saying it Equivocality: Organizational information theory Etymological fallacy: A kind of linguistic misconception Evasion (ethics): Tell the truth while deceiving False equivalence: Fallacy based on flawed reasoning If-by-whiskey: An example Map-territory relation: Concept that words used to describe an underlying reality are arbitrary abstractions not to be confused with the reality itself Mental reservation: A doctrine in moral theology No true Scotsman: Changing a definition to exclude a counter-example Persuasive definition: Skewed definition of term Plausible deniability: A blame-shifting technique Polysemy: The property of word or phrase having certain type of multiple meanings Principle of explosion: One of the fundamental laws in logic Syntactic ambiguity, Amphiboly, Amphibology: Ambiguity of a sentence by its grammatical structure When a white horse is not a horse: An example

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