Definition:A Fortiori

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Definition

A fortiori knowledge arises from stronger facts already established.

An a fortiori argument is most commonly used by applying a general fact to a particular case.


Examples

Prime Number

The following is an example of a fortiori reasoning:

The number $11$ is a prime number.
Hence $11$ a fortiori is not divisible by $5$.


A Family Matter

The following is an example of a fortiori reasoning:

I am too old for this game; therefore, my father is also too old.

That is, the knowledge:

My father is too old for this game.

arises a fortiori from the stronger knowledge that:

I am too old for this game.


Also see


Linguistic Note

The phrase a fortiori derives from the Latin for from the stronger, that is, with even stronger reason.


Sources