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A fortiori is a Latin term used in legal parlance, which translates to with stronger reason or even more so. It is typically used in legal reasoning to denote that if one fact is certain then one can infer that a second fact is even more certain.
Lawyers use this method of reasoning to argue that if something applies in one situation, then it should apply even more so in another situation. It’s often used to make a point about the strength of a claim or argument.
For example, a lawyer might use an a fortiori argument in a case where a defendant is charged with both assault and jaywalking. If the evidence is sufficient to prove that the defendant committed assault (a more serious crime), then, a fortiori, it should be more than sufficient to prove that he was jaywalking (a less serious crime).