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A fallacy of necessity is a fallacy in the logic of a syllogism whereby a degree of unwarranted necessity is placed in the conclusion. A fallacy is a component of an Argument which being demonstrably flawed in its Logic or form renders the argument invalid in whole A syllogism, or logical appeal, (συλλογισμός &mdash "conclusion" "inference" (usually the categorical syllogism) is a kind of

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. In Rhetoric, a tautology is an unnecessary (and usually unintentional repetition of meaning using different words that effectively say the same thing twice (often originally 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.


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