Argumentum a fortiori is a form of argumentation that draws upon existing confidence in a proposition to argue in favor of a second proposition that is held to be implicit in, and even more certain than, the first.
Examples
If a person is dead, then one can with equal or greater certainty argue a fortiori that the person is not breathing. "Being dead" trumps other arguments that might be made to show that the person is dead, such as "he is no longer breathing"; therefore, "he is no longer breathing" is an extrapolation from his being dead and is a derivation of this strong argument.
If driving 10 mph over the speed limit is punishable by a fine of $50, it can be inferred a fortiori that driving 20 mph over the speed limit is also punishable by a fine of at least $50.
If a teacher refuses to add 5 points to a student's grade, on the grounds that the student does not deserve an additional 5 points, it can be inferred a fortiori that the teacher will also refuse to raise the student's grade by 10 points.
Usage
In Garner's Modern American Usage, Garner says writers sometimes use a fortiori as an adjective as in "a usage to be resisted". He provides this example: "Clearly, if laws depend so heavily on public acquiescence, the case of conventions is an a fortiori one." A fortiori arguments are regularly used in Jewish law under the name kal va-chomer, literally "mild and severe", the mild case being the one we know about, while trying to infer about the more severe case. In ancient Indian logic, an inference derived from an a fortiori postulation is known as kaimutika or kaimutya nyaya, from the wordskim uta meaning "even more so". In Islamic jurisprudence, a fortiori arguments are proved utilising the methods used in qiyas.
Types
''A maiore ad minus''
In logic, a maiore ad minus describes a simple and obvious inference from a claim about a stronger entity, greater quantity, or general class to one about a weaker entity, smaller quantity, or specific member of that class:
The reverse, less known and less frequently applicable argument is a minore ad maius, which denotes an inference from smaller to bigger.
In law
“Argumentum a maiori ad minus” – works in two ways:
“who may more, all the more so may less” and relates to the statutory provisions that permit to do something
“who is ordered more, all the more so, is ordered less” and relates to the statutory provisions that order to do something
An a fortiori argument is sometimes considered in terms of analogical reasoning – especially in its legal applications. Reasoning a fortiori posits not merely that a case regulated by precedential or statutory law and an unregulated case should be treated alike since these cases sufficiently resemble each other, but that the unregulated case deserves to be treated in the same way as the regulated case in a higher degree. The unregulated case is here more similar to the regulated case than this case is similar to itself.
