Fallacy of Necessity

fellacia necessitas

Description: A logical fallacy in a syllogism where necessity is stated in the conclusion and necessity is not stated in both the premises.

Logical Form:

A is necessarily B.

C is A.

Therefore, C is necessarily B.

Example #1:

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).

Example #2:

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.

Exception: None.