Description: A logical fallacy in a syllogism where necessity is stated in the conclusion and necessity is not stated in both the premises.
A is necessarily B.
C is A.
Therefore, C is necessarily B.
Mothers necessarily have children.
Fran is a mother.
Therefore, Fran necessarily has children.
Explanation: It is clear that to be a mother, you must have children (at least one -- biological or adopted, doesn’t matter for this example). Fran is a mother -- so far so good. But the conclusion is not true -- Fran does not have to have children simply because she does not have to be a mother. To escape this fallacy, we could change our second premise to, “Fran is necessarily a mother” (whatever that may mean).
Synchronized swimming fans necessarily love synchronized swimming.
Momo is a synchronized swimming fan.
Therefore, Momo necessarily loves synchronized swimming.
Explanation: We have the same exact form as example #1, but with different content. Momo would only have to (necessarily) love synchronized swimming, if and only if, he was necessarily a synchronized swimming fan, which we cannot assume he is because it was not stated. This argument is invalid and fallacious.