- Petitio principii (Latin: "pursuit/attack of the source")
"Proving" that something is true by taking your conclusion as one of your premises, usually done implicitly rather than explicitly. Few people are fooled by having your conclusion as your only premise, as in "Joe is mad at Jill, therefore Joe is mad at Jill." In rhetoric, such arguments are called tautologies, and they're essentially a pretty but meaningless way of saying the same thing twice. Logic, meanwhile, has its own form of tautology: a statement or chain of statements which are sound, valid, and true under any condition.note Begging the question is what happens when you confuse the two.
Put broadly, this fallacy applies to any argument where one or more premises are at least as contentious as the conclusion itself, and for the same reasons, such as:
- Alice says she is honest.
If an honest person says something, it must be true.
Therefore Alice is an honest person, because an honest person says so.
"Begging the question" is often used to mean "leading inevitably to the question" (similar to how something can be said to "invite criticism" or someone can be "asking for a smack") in popular media, though this usage is a common Berserk Button for academics aware of the original use noted above (it doesn't help that the academic meaning is not only horribly counter-intuitivenote , but also a mistranslation; as stated above, the original Latin means something more like "arguing the source"). Nevertheless, to avoid confusion, this fallacy is sometimes referred to by its Latin name, petitio principii, in more formal settings.
See also No True Scotsman. If both parties accuse each other of begging the question, then it's a philosophical impasse — no one can agree on a common set of premises; compare Values Dissonance. See also Tautological Templar, who believes that since they are good, everything they do is justified as good.
- A classic Bill Mauldin political cartoon from the early 1960s features an American soldier standing in a field of large cartoon mushrooms, a reference to the conical hats worn by Asian field workers and guerrillas, explaining to a comrade the challenge of ground combat in Vietnam: "You got your mushrooms and your toadstools. The mushrooms are harmless, the toadstools will kill you. You'll know it's a toadstool if it kills you."note
- The Discworld Companion entry for the Ephebian philosopher Expletius says that while he proved the Disc was 10,000 miles in diameter, all philosophers had their own "proof" of the Disc's size giving wildly different numbers, but "they turned out to be wrong". With how it "turned out" they were wrong carefully elided, the suggestion is we're taking the actual diameter of the Disc as established, and judging the proofs based on that.
- The Roundworld inspiration for this is mentioned in The Science of Discworld: Eratosthenes proved the circumference of the Earth was 252,000 stadia. This is very accurate, assuming you use the right kind of stadium (there were several different measurements of that name). How do we know which one he used? Well, nowadays we do know the circumference of the Earth. So if that's 252,000 stadia, then...
- In Umineko: When They Cry, accepting the red text as only speaking the truth requires you believe both that Beatrice is being honest and that the red text speaks only the truth when statements like "The red text speaks only the truth!" come up. Well, at least that's the case until we see Battler attempt to use the red to say something that turns out to be untrue. That said, it does happen to be true: Anything said in red is at worst misleading.
- Antimony uses almost this exact argument in this Gunnerkrigg Court strip. Of course, it's rather justified, as it's against robots who have trouble understanding this fallacy.
- In the days of Usenet, in a football forum, one poster postulated that you need a great coach to win a Super Bowl. He then defined a "great coach" as one who had won a Super Bowl.