Definition:A Fortiori

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.

The following is an example of a fortiori reasoning:

The number $11$ is a prime number.

Hence $11$ a fortiori is not divisible by $5$.

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.

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