Fallacy of necessity
From Wikipedia, the free encyclopedia
A fallacy of necessity is a fallacy in the logic of a syllogism whereby a degree of unwarranted necessity is placed in the conclusion.
Example:
- a) Bachelors are necessarily unmarried.
- b) John is a bachelor.
- Therefore, c) John cannot marry.
The condition a) is a tautology and therefore valid on its face. The condition b) is a statement of fact about John which makes him subject to a); that is, b) declares John a bachelor, and a) states that all bachelors are unmarried.
Because c) presumes b) will always be the case, it is a fallacy of necessity. John, of course, is always free to stop being a bachelor, simply by getting married; if he does so, b) is no longer true and thus not subject to the tautology a). In this case, c) has unwarranted necessity by assuming, incorrectly, that John cannot stop being a bachelor.
