Equivocation

In logic, equivocation^[a] is an informal fallacy involving using a certain word in multiple contexts within an argument.^[1][2] It is a type of ambiguity coming from a phrase with two or more meanings ‒ rather than the grammar of the sentence.^[1]

Fallacy of four terms

Equivocation in a syllogism^[b] produces a fallacy of four terms^[c]. Below is an example:

Since only man [human] is rational.

And no woman is a man [male].

Therefore, no woman is rational.^[1]

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

Motte-and-bailey fallacy

Main article: Motte-and-bailey fallacy

The motte (raised area) and bailey (walled courtyard) defenses at Launceston Castle

Equivocation can involve equating two similar positions, one modest and easy to defend, and the other more controversial. The arguer then promotes the controversial one, while insisting that they are advancing the more modest one when being criticized.

Footnotes

1. Meaning "calling two different things by the same name"
2. A chain of reasoning
3. quaternio terminorum