Equivocation ("to call by the same name") is classified as an informal logical fallacy. It is the misleading use of a term with more than one meaning or sense (by glossing over which meaning is intended at a particular time). It generally occurs with polysemic words (words with multiple meanings).
It is often confused with amphiboly; the difference is that equivocation arises from an ambiguous definition of a word, while amphiboly refers to ambiguous sentence structure due to punctuation or syntax.
Equivocation is the use in a syllogism (a logical chain of reasoning) of a term several times, but giving the term a different meaning each time. For example:
- A feather is light.
- What is light cannot be dark.
- Therefore, a feather cannot be dark.
In this use of equivocation, the word "light" is first used as the opposite of "heavy", but then used as a synonym of "bright" (the fallacy usually becomes obvious as soon as one tries to translate this argument into another language). Because the "middle term" of this syllogism is not one term, but two separate ones masquerading as one (all feathers are indeed "not heavy", but it is not true that all feathers are "bright"), this type of equivocation is actually an example of the fallacy of four terms.
The fallacy of equivocation is often used with words that have a strong emotional content and many meanings. These meanings often coincide within proper context, but the fallacious arguer does a semantic shift, slowly changing the context by treating, as equivalent, distinct meanings of the term.
In English language, one equivocation is with the word "man", which can mean both "member of the species, Homo sapiens," and "male member of the species, Homo sapiens." The following sentence is a well-known equivocation:
- "Do women need to worry about man-eating sharks?", in which "man-eating" is construed to mean a shark that devours only male human beings.
- "People testing for faster than light phenomena must not know it's impossible to travel faster than the speed of light."
Here, "impossible" is construed to mean that there's zero chance that relativity (the physics theory) is wrong. However, "it's impossible to travel faster than the speed of light" is taught to mean that based on relativity, there's no way (hence "impossible") to travel faster than the speed of light.
This occurs where the referent of a word or expression in a second sentence is different from that in the immediately preceding sentence, especially where a change in referent has not been clearly identified.
- All jackasses have long ears.
- Carl is a jackass.
- Therefore, Carl has long ears.
Here the equivocation is the metaphorical use of "jackass" to imply a stupid or obnoxious person instead of a male donkey.
"Nothing is better than"
- Margarine is better than nothing.
- Nothing is better than butter.
- Therefore, margarine is better than butter.
Specific types of equivocation fallacies
- Evasion (ethics)
- Fallacy of four terms
- Mental reservation
- Plausible deniability
- When a white horse is not a horse
- Logical Fallacy: Equivocation The Fallacy Files