This fallacy occurs in a categorical syllogism when the syllogism has four terms rather than the requisite three (in a sense, it cannot be a categorical syllogism to begin with!) If it takes on this form, it is invalid. The equivocation fallacy can also fit this fallacy because the same term is used in two different ways, making four distinct terms, although only appearing to be three.
Examples:
- All X are Y. All A are B. Therefore, all X are B.
Synonyms: ambiguous middle term