A fortiori argument

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The Latin phrase argumentum a fortiori literally means one of the following:

  • "from the stronger"
  • "even more so"
  • "with even stronger reason"

It denotes a proof of a claim by means of an already proved stronger claim. For example, if it is forbidden to ride a bike with an extra passenger, then it is also forbidden to ride a bike with two extra passengers. Or, if one can lift a 100 lb object, then it follows that one can lift a 50 lb object.

There are two types of the a fortiori argument:

The a fortiori argument is most often used in order to reinforce a claim, though sometimes also to incorrectly justify a claim taking it as a premise (petitio principii).

[edit] References

  • Grabenhorst, Thomas K.: Das argumentum a fortiori, Verlag „Peter Lang“ 1990 ISBN-Nummer 3-631-43261-5
  • Schneider: Logik für Juristen, S. 158ff.