Fallacy of necessity
Encyclopedia
A fallacy of necessity (fellacia necessitas) is a fallacy in the logic of a syllogism
whereby a degree of unwarranted necessity is placed in the conclusion.
Example:
The condition a) appears to be a tautology
and therefore true. 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. Formally speaking, this type of argument equivocates between the de dicto necessity of a) and the de re necessity of c). The argument is only valid if both a) and c) are construed de re. This, however, would undermine the argument, as a) is only a tautology de dicto--indeed, interpreted de re, it is false.
Syllogism
A syllogism is a kind of logical argument in which one proposition is inferred from two or more others of a certain form...
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) appears to be a tautology
Tautology (rhetoric)
Tautology is an unnecessary or unessential repetition of meaning, using different and dissimilar words that effectively say the same thing...
and therefore true. 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
Tautology (logic)
In logic, a tautology is a formula which is true in every possible interpretation. Philosopher Ludwig Wittgenstein first applied the term to redundancies of propositional logic in 1921; it had been used earlier to refer to rhetorical tautologies, and continues to be used in that alternate sense...
a). In this case, c) has unwarranted necessity by assuming, incorrectly, that John cannot stop being a bachelor. Formally speaking, this type of argument equivocates between the de dicto necessity of a) and the de re necessity of c). The argument is only valid if both a) and c) are construed de re. This, however, would undermine the argument, as a) is only a tautology de dicto--indeed, interpreted de re, it is false.