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(Fallacies of) Ambiguity

A number of misconceptions and misunderstandings can arise from the fact that a term, a statement, the understanding of a concept, or even the way it is referred to is ambiguous.

For example Open in Syllogism-Finder App:

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

Obviously, something different is meant by “light” in the first premise than in the second. The conclusion is therefore invalid.



The best known and probably also the most common form of a fallacy of ambiguity is based on an equivocation – as in the example above, which also shows how such an equivocation can lead to an invalid conclusion (in this case, a so-called “four-term fallacy”).

However, ambiguity is not always as easy to recognize as in the example. Especially in the case of rather abstract terms, these can remain undetected and lead to equally subtle and difficult-to-detect errors.

The classic example is the following (invalid) syllogism (loosely based on Thomas Aquinas):

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

In this, the term “human” refers first to the species as a whole and once to the members of this genus. The difference may be subtle, but it leads to an invalid conclusion here.


But not only the terms in themselves can be ambiguous, ambiguity can also arise from the syntactic structure of the statement. A simple example of this are statements like:

The English are tea drinkers.

Although at first glance this resembles a logical categorical statement, as it is typically used in syllogisms, it lacks a quantifier, i.e., a specification of whether the statement refers to all or to part of the conceptual set. Such ambiguous statement forms are known as Generic generalizations.

These are however only one example of a whole group of ambiguities that can arise from the structure of the statement (so-called “amphibolies”). These are described here under Syntactic ambiguity.

Ambiguous concepts

Another form of ambiguity exists when two related but distinct concepts are conflated. This is referred to as the Motte-and-Bailey fallacy. Such mixups can be used as an unfair discussion tactic, but more typically the reason for the confusion is simply that they are difficult to distinguish and it is thus a simple mistake.

Intensional ambiguity

All the above forms refer to ambiguities in the things or concepts that are referred to by the used expressions, i.e. their “extensions”. There is, however, also another aspect of a statement that has a potential for ambiguity, i.e the intensions, i.e. the way in which the extension is referred to.

Ambiguities can also result from different intensions of a statement, which will be described here under Intensional fallacy.


See also

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