A Fortiori

The Latin term a fortiori is used to refer to an argument made from a previously made argument or previously proven fact. The original argument is considered to be stronger, or to come from a stronger basis, than the second argument, therefore, the arguer presents the original claim to support his own, weaker claim. In making a legal argument, the term a fortiori is most commonly used as an adverbial phrase. To explore this concept, consider the following a fortiori definition.

Definition of A Fortiori


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  1. For an even stronger reason


  1. With more convincing force


1561    Latin  fortiori argumento  (literally, “from the stronger,” as in an argument)

Use of A Fortiori

The term a fortiori is sometimes used in legal writing to say that an argument being made has already been expressed from an even stronger position. For instance, the writer may state that, because a particular fact is true, the court can infer that his own argument, or second fact, is also true. A simple example of a fortiori would state: “My wife loves turquoise, so she will love any piece of turquoise jewelry I give her.” Another, more legal example, might be “Allen died on April 22nd, therefore, a fortiori, Allen did not murder Joe on April 23rd.”

As another example of a fortiori, if Matt’s math teacher proves to the class that 2 is less than 10 by putting 10 oranges on a table, then taking away 8 of them, he has made is first point. Once this is understood by the class, it is an a fortiori conclusion that 2 is also less than 20. This logical comparison is not actually between physical items, however, but between the relative values, principles, or rules. In other words, it has nothing to do with oranges, but with the concept of relative values. Two is less than 10, and therefore less than 20 anything.

In making an a fortiori comparison, the issues must be of like things, sharing essential factual elements. For instance, if Amy does not trust her son to ride his bicycle to school, it does not necessarily mean she can’t trust him to carry in the groceries and put them away. These two tasks are not alike, and cannot be compared by a fortiori.

On the other hand, this example of a fortiori might be similar enough for comparison if some additional information was available. For instance, if Amy cannot allow her son to ride his bicycle without supervision because he has frequent seizures, which could cause him to crash, that would be more than enough reason to not allow him to drive a car. Therefore, Amy cannot allow her son to ride his bicycle because he has seizures, therefore it follows, a fortiori, that she cannot allow her son to drive a car. In this example of a fortiori, the two things compared are similar in light of the additional information about the son’s seizures.

A Fortiori Example

Amanda gets out of her apartment right on schedule to get to work five minutes ahead of time. As she makes her way to the parking lot, she sees that all four of the brand new tires have been stolen off her car. Amanda’s boss is a stickler for punctuality, having zero tolerance for tardiness at work. Amanda cannot find a friend who is able to pick her up and take her to work at the moment, and a call to the taxi company places a ride at 30 minutes out.

Amanda calls her boss to tell him what’s happened, and that she will be in late. He isn’t happy, but what can she do? When Amanda finally arrives just over an hour late for her shift, her boss accuses her of not having a good reason for being late, saying that he doesn’t believe the story about her tires. He fires her on the spot. Amanda has had a stellar performance record in her job, having never been late, never taken a sick day, and doing her duties with great efficiency.

Amanda’s father, who is an attorney, helps Amanda take her case to the labor board. In the written complaint, he includes a copy of the police report documenting the theft, and argues that, because it is known that Amanda’s tires were indeed stolen, a fortiori, the board can conclude that she didn’t lie to her employer, and that she had a good reason for being late for work that day.

Related Legal Terms and Issues

  • Legal Argument – The reasons offered by a party for or against something; an oral or written presentation of such reasons, intended to convince or persuade the court, or a jury.
  • Labor Board – A governmental agency that regulates laws in the workplace. This may refer to the National Labor Relations Board, or to agencies in each state, working to monitor and improve working conditions, and fair labor practices.

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