Modal Scope Fallacy
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A fallacy of necessity is a
fallacy A fallacy is the use of invalid or otherwise faulty reasoning, or "wrong moves," in the construction of an argument which may appear stronger than it really is if the fallacy is not spotted. The term in the Western intellectual tradition was int ...
in the logic of a
syllogism A syllogism ( grc-gre, συλλογισμός, ''syllogismos'', 'conclusion, inference') is a kind of logical argument that applies deductive reasoning to arrive at a conclusion based on two propositions that are asserted or assumed to be true ...
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 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. Using the formal symbolism in modal logic, the ''de dicto'' expression \Box (Bx\rightarrow\neg Mx) is a tautology, while the ''de re'' expression Bx\rightarrow \Box\neg Mx is false.


See also

* ''De dicto'' and ''de re'': Context of modality * Modal logic


References

* * * * {{Formal Fallacy Necessity Necessity