All the chewy goodness of fallacious reasoning in new fun-sized bites!
Petitio Principii is Latin for “laying claim to a principle”
In A Preface to Philosophy author Mark B. Woodhouse defines the fallacy of petitio principii as: “An argument [that] begs the question… when it assumes the truth (or falsity) of the very claim it is supposed to prove (or disprove).” Woodhouse lists two important forms of petitio principii or question-begging arguments:
Circular Arguments: Circular arguments are sometimes not easily identified. Essentially, the conclusion of the argument is disguised as a premise in order to arrive at the conclusion. For example, “The doctrines of Christianity are incoherent because they do not agree with each other.” Look at the statement again: “(Conclusion) The doctrines of Christianity are incoherent because (Premise) they do not agree with each other.” If something is “incoherent” it means that it does not agree with itself. Therefore, the statement above is really saying, “(Conclusion) The doctrines of Christianity are incoherent because (Conclusion) the doctrines of Christianity are incoherent.”
Question-Begging Definitions: According to Woodhouse question-begging definitions “ensure that a conclusion will be true by using definitions that rule out any other possibility.” Consider this example: Ralph says, “No true Christian would be against same-sex marriage.” Paul points out that Michael is a Christian and against same-sex marriage. Ralph says, “This just shows that Michael is not a true Christian.” By redefining “Christian” to mean anyone who supports same-sex marriage, Ralph has ensured that his original claim is true.
The best way to point out someone’s fallacy is by using the Columbo tactic as laid out in the Funsized Tactics series. There are plenty of excellent resources on formal and informal fallacies available in book, e-book, or PDF format. Two great places to start are A Preface to Philosophy by Mark B. Woodhouse and Schaum’s Outlines: Logic by John Nolt, Dennis Rohatyn, and Achille Varzi.
 Mark B. Woodhouse, A Preface to Philosophy (Belmont: Wadsworth, Cengage Learning, 2006), 81.