Definition:A Fortiori

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A fortiori knowledge arises from stronger facts already established.

For example, in:

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

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.

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