How a fortiori Works

Translated from Latin, a fortiori means “with even stronger reason.” Though it is a term reserved for law, we use the concept of a fortiori more than we think we do. A fortiori works to emphasize a point is true by presenting evidence that very obviously backs it up. People most use this concept when they want to make a point about somebody’s finances and buying power, use of time, or work ethic. The layman more often uses it to accuse someone of something negative or unappealing.

A fortiori Example

There are plenty of examples of a fortiori. However, unless you’re in court or using it ironically, you’d very rarely ever say “a fortiori.” Let’s say you are talking with a friend about their partner. Your friend’s biggest complaint about their partner is that they cannot seem to save money to pay their portion of the bills. When it comes time to contribute their share, it’s always, “Oh, I don’t have any money.” You scoff at this and come back with, “If he has enough money to get a full sleeve of tattoos, then, a fortiori, he has enough money to pay his portion of the rent.”

For another example of a fortiori, let’s look at a character trait instead of money. A man is accused of stealing from his mother. The plaintiff, in his closing remarks, reminds the jury that he has, in the past, also stolen from his sister, grandfather, and close friends. “If he stole from his family before, then, a fortiori, he would have no problem stealing from his family again.”

You could potentially use a fortiori in a joking way, too. If you’re working on homework with a friend and come up with the same answers, you’ve set yourself up for an a fortiori moment. In class, the teacher is reviewing the homework and calls on your friend to share her answer. It’s incorrect. The teacher calls on you next. Instead of saying the same thing, you say, “If she’s wrong then, a fortiori, so am I.”

Significance of a fortiori

A fortiori is much more powerful than just using “therefore.” In court, lawyers use a fortiori to show that they are certain, that their conclusions come from a certain premise; they are not just a result of a series of events. Yes, a fortiori is still an inference, but it is a confident one that has a strong reason behind it.

A fortiori vs. A priori

They may rhyme, but a fortiori and a priori deal with two different things. A fortiori follows a presentation of evidence in order to prove an inference. A priori describes a type of knowledge and information based on deduction as opposed to observation. Although you can argue that a fortiori is knowledge based on deduction, too, the concept is frequently used in an observational way outside of the courtroom, casually.