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Use of a term in multiple different meanings. Equi­voca­tions are a possible source of fallacies of ambiguity, as well as mis­under­stand­ings and a rhetorical device of confusion, as in the following example:

Gandalf: Have you been eaves­dropping?
Sam: I haven’t dropped no eaves, Sir, honestly!

In this movie quote, Sam is trying to using the term “eaves­dropping” (or “dropping eaves”) with a very different meaning than Gandalf. Indeed it appears as if he is deliberately confusing the meaning in order to talk himself out of an unpleasant situation.

An example of an equivocation in logical statements could be the following :

Nothing light can ever be dark.
All feathers are light.
Therefore: no feather can be dark.

The term “light” is used here in two different meanings here: in the major premise is stands for the opposite of “dark”, but for the opposite of “heavy” in the minor one. By equivocation of the term, this syllogism specifically commits the fallacy of the ambiguous middle term.

Equivocation of abstract terms

While such ambiguities are usually easily identified as synonyms when they refer to actually existing things or perceivable properties, this distiction can be rather difficult for abstract terms

use of synonyms for terms that stand for real life objects is usually quite easy to understand, the distinction for abstract concepts can sometimes be quite difficult, as the following example (loosely based on Thomas Aquinas) shows:

Humans are a species.
Socrates is a human.
Therefore: Socrates is a species.

Here, the term “human” in the major (first) clause is used as a generic term, i.e. it refers to the genus “human” as a whole, while the same term in the minor (second) clause refers specifically to the individuals of that genus. One could thus rephrase the latter as: “Socrates is an individual of the human genus” (see also:  semiotic fallacy).

Since these two meanings have different extensions, this is a case of equivocation and thus it commits the fallacy of the ambiguous middle term.

In principle, such fallacies of ambiguity are easiest to commit if the terms used are rather complex, abstract, vague – and possibly even contra­dictory defined ( weakly defined terms).

Ambiguities in concepts and positions

Not only individual words or terms can be con­founded due to ambi­gui­ties, but also com­plex con­cepts or posi­tions can be affected by this. It is also valid that the more ab­stract and com­pli­cated they are, the more diffi­cult it can be to re­cog­nise ambiguities.

An example of systematic exploitation of such ambiguities as an unfair discussion tactic is known as  “motte-and-bailey fallacy”.

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