fallacy

Equivocation

Also called: Ambiguity fallacy

Equivocation uses one word in two different senses within the same argument, so the conclusion seems to follow when it does not.

How it works

The argument relies on a word meaning one thing in a premise and something else in the conclusion. Because the word is the same, the switch goes unnoticed.

Examples

"Evolution is just a theory, and theories are only guesses."

"Theory" in science means a well-tested explanation, not a guess.

"The law says all men are equal, but men are clearly not equal in height or strength."

Legal equality and physical equality are different meanings of "equal".

How to spot it

A key word carries the whole argument.
Try substituting a precise definition in each place it appears.
The conclusion sounds surprising given modest premises.

How to respond

Ask what the word means at each step.
Restate the argument with precise terms and see whether it still holds.
Agree on definitions before debating the conclusion.