Petitio Principii (Begging the Question)


The words and phrases used to express the premisses are synonymous with the words and phrases used to express the conclusion. That is, the conclusion is merely a re-wording of the premisses.



The Latin term "petitio principii" is translated literally into English as "begging the question." "Petitio" means to petition, or to appeal to, or to beg; "principii" is the principle which the reasoning seeks to explore, i.e. the issue in question. Recently the phrase "to beg the question" has taken on a different meaning. In a conversation (or, especially, in an interview) the answer to one question may lead to or suggest a follow-up question. In that case, the questioner may introduce the suggested question by saying, "Well, that begs the question..." Since common usage is the only criterion for correct usage, it is pointless to complain that this new usage is incorrect. However, it is worth pointing out that this new usage is probably due to a misunderstanding of the original phrase as it has been traditionally used by logicians.



"No one is permitted to use the gymnasium on weekends, since people are permitted to use the gymnasium only on week days."


"The reason I keep insisting that there was a relationship between Iraq and Saddam and al-Qaida is because there was a relationship between Iraq and al-Qaida." - George W. Bush, (2004, at a press conference defending the war in Iraq.)



Petitio Principii is undeniably a common fallacy. I often see it in student papers, for example, where it often takes the form of an argument by double negation: "My position is not false, therefore it is true." One characteristic of the Petitio Principii fallacy is that it is more likely to fool the person making the argument than the person he or she is trying to persuade. When one is already persauded of the truth of a position, it is easy to mistake a re-statement or re-affirmation of that position as an argument for that position. Perhaps the single most important thing that students should learn from a philosophy class is the difference between holding or believing a position and being able to justify that position, i.e. the difference between the position itself and the arguments for that position. The Petitio Principii fallacy can easily fool people who do not yet understand this distinction.


Source: Aristotle, Sophistical Refutations 5 (167b: 1 - 15).


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