Quaternio terminorum
Quaternio terminorum
In the categorical syllogism (logic, formal, 5), the major and minor premisses must have a term in common, the middle term. Violation of this rule is the fallacy of quaternio terminorum, or of four terms. It is most apt to arise through equivocation (q.v.), an ambiguous word or phrase playing the role of the middle term, with one meaning in the major premiss and another meaning in the minor premiss; and in this case the fallacy is called the fallacy of ambiguous middle. — A.C.