Taxonomy: Logical Fallacy > Informal Fallacy > Vagueness

Subfallacy: The Fallacy of the Heap


Vagueness is a characteristic of language, specifically of those words and phrases that classify or qualify objects, that is, common nouns and adjectives. Such a word or phrase divides the world of objects into those it applies to1, and those to which it doesn't apply. For example, the common noun "elephant" divides the world into elephants and non-elephants.

What characterizes a vague term is the existence of borderline cases which do not clearly belong or not belong to its extension. For example, consider the familiar common noun "chair": some things are clearly chairs—what you're sitting on right now, for instance—and others are clearly not—for instance, an elephant, even though you might sit upon one. But there are many borderline cases: barstools, beanbag "chairs", school desks, etc.

The fallacy of Vagueness comes about when the appearance of cogency of an argument depends upon vagueness in its terms.

The mere fact of vagueness is not sufficient to justify an accusation of fallacy, but it is sometimes a logical boobytrap which can cause the unsuspecting person to fall into fallacious reasoning. For this reason, it is useful to be aware of and on our guard against vague terms, so that we can continue to use our vague language without being ensnared by it.

As a logical fallacy, Vagueness is the most general fallacy in which the fallaciousness of an argument turns on vague language. This is why no Example is given for this fallacy, as instances should be examples of a subfallacy.



  1. The "extension" of the term.
  2. For a typical example, see: S. Morris Engel, With Good Reason: An Introduction to Informal Fallacies (6th Edition, 2000), pp. 74-5.
  3. The opposite of a subfallacy, that is, a more general form of a specific type of fallacy.
  4. Though currently there is only one subfallacy of Vagueness―see above―this may change in the future.