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A fortiori is a Latin term that can be translated as by or from the stronger reason. The term is used in a variety of disciplines including in studies of logic and rhetoric. When used in law, it’s often a way of setting up an inference that can be drawn from a strong and indisputable fact: that if one thing is true another thing must also be true.
An example of the way an a fortiori argument works can be of great use in understanding the idea. Two gentleman were sitting on a bench together on the night of a robbery and one of them, Mr. Smith, has since been accused of committing the robbery. It can’t be directly established that Mr. Smith didn’t commit the crime, but it can be established that the gentleman with him, Mr. Jones, didn’t. It turns out the distance to the robbery site is too far for Mr. Jones to have run in order to commit the crime at the time police established it occurred.
Mr. Jones is very fit and athletic, and even with these characteristics the distance was too far away. It can thus be established that Mr. Jones couldn’t and didn’t commit the crime. Mr. Smith has a limp, weighs 400 pounds, and uses a cane. If Mr. Jones couldn’t commit the robbery, Mr. Smith certain couldn’t. There is no way he could get to the robbery site in time. An a fortiori argument is thus established. One proposition backs up the other and both become true.
Other examples of a fortiori include inferences drawn about many different types of things. The fact that 13 is less than 20, means 13 is also less than 100 because it’s recognized that 20 is less than a hundred. A person younger than his sister is also younger than anyone else his sister’s age or older. Essentially, when one thing is true and accepted, it can make other things true, too.
In context of the law, these inferences can be very useful because they can help lawyers, juries, or judges draw other conclusions based on evidence presented. An excellent fictional example of this is in the book, To Kill a Mockingbird. Atticus Finch makes an a fortiori argument that Tom Robinson could not have beaten Mayella Ewell because the hits would have had to come from a left-handed person, and Tom had no use of his left hand.
When it is hard to establish direct evidence of someone’s innocence or guilt, often the next best thing is to look around the situation for other evidence that creates a clear and logical inference. Finding something that makes something else true or false is of great use. Many instances exist where something is proved or disproved from the stronger reason, or a fortiori.