Ambiguous middle explains itself. It is the fallacy of giving to the middle term a double meaning. In form the argument may contain but three terms, yet in meaning there are in reality four terms. For this reason ambiguous middle and the fallacy of four terms appear to be about one and the same thing; but in this treatment we shall regard them as mutually exclusive, and this is the distinction:
Invalid arguments of “ambiguous middle” have only three terms in form but four terms in meaning. This signifies that the middle term though identical in form is given a double meaning.
Invalid arguments of “four terms” always have four terms in both form and meaning; they are “logical quadrupeds” in every sense of the word.
ILLUSTRATIONS.
Ambiguous middle.
(a) “Necessity is the mother of invention,”
Bread is a necessity,
∴ Bread is the mother of invention.
(b) “Nothing is better than wisdom,”
Dry bread is better than nothing,