A fortiori argument
An a fortiori argument draws upon existing confidence in a proposition to argue in favor of a second proposition that is held to be implicit in the first. The second proposition may be considered "weaker," and therefore the arguer adduces a "stronger" proposition to support it.
For example, if a person is dead (the stronger reason), then one can with equal or greater certainty argue that the person is not breathing. "Being dead" trumps other arguments that might be made to show that the person is not breathing, such as, for instance, not seeing any sign of breathing. (Provided, of course, that the determination of "being dead" can be made without using "not breathing" as evidence, else this scenario becomes circularly reasoned.)
In the English language, the phrase a fortiori is most often used as an adverbial phrase meaning "by even greater force of logic" or "all the more so".
Bryan Garner, an authority on usage, has written in Garner's Modern American Usage that writers sometimes use a fortiori as an adjective, which he says is "a usage to be resisted." As an example of this he gives the sentence, "Clearly, if laws depend so heavily on public acquiescence, the case of conventions is an a fortiori [read even more compelling] one."
In ancient Indian logic (nyaya), an a fortiori inference is known as kaimutika or kaimutya nyaya, from the words kim uta meaning "even more so."
- Morwood, James (1998). A Dictionary of Latin Words and Phrases. Oxford: Oxford University Press. pp. x–xii. ISBN 978-0-19-860109-8.
- Garner, Bryan A. (2009). Garner's Modern American Usage (3rd ed.). Oxford: Oxford University Press. p. 28. ISBN 978-0-19-538275-4.
- Hallaq, Wael (2009). Sharī'a: Theory, Practice, Transformations (1st ed.). Cambridge: Cambridge University Press. p. 105. ISBN 0521678749.
- Grabenhorst, Thomas K.: Das argumentum a fortiori, Verlag „Peter Lang“ 1990 ISBN 3-631-43261-5.
- Schneider: Logik für Juristen, pages 158ff.