A fortiori argument
An a fortiori argument / / is an "argument from a yet stronger reason." For example, if it has been established that 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.
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.
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 because." It is sometimes used as an adjective, but this usage is deprecated by lexicographer Bryan Garner, an authority on usage.
An a fortiori argument is most often adduced in order to reinforce a claim already asserted by other means.
If an argument's proponent attempts to reinforce their point by adducing a stronger argument, they should take care that the relevant portion of the stronger argument does not rely upon the claim that is to be reinforced. If it does, the attempt to reinforce will fail, being an instance of the fallacy known as petitio principii ("begging the question").
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."
- Grabenhorst, Thomas K.: Das argumentum a fortiori, Verlag „Peter Lang“ 1990 ISBN 3-631-43261-5.
- Schneider: Logik für Juristen, pages 158ff.