A fallacy of necessity is a fallacy in the logic of a syllogism whereby a degree of unwarranted necessity is placed in the conclusion. It has been suggested that this article or section be merged with fallacy. ... A syllogism (Greek: — conclusion, inference), usually the categorical syllogism, is a kind of logical argument in which one proposition (the conclusion) is inferred from two others (the premises) of a certain form. ...
Example:
Bachelors are necessarily unmarried.
John is a bachelor.
John is necessarily unmarried.
This example seems watertight, but the problem lies with the necessarily in c). c) suggests that it is inconceivable for John to marry - however b) does not state this; merely that, at present, John happens to be a bachelor. For c) to hold true, both a) and b) would have to be necessarily true, but only a) is, since it is a tautology. Within the study of logic, a tautology is a statement containing more than one sub-statement, that is true regardless of the truth values of its parts. ...
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