Begging the question: An argument whose conclusion is assumed in the premises.
Circular argument: An argument whose premises and conclusion support each other, forming a logical circle.
I have included a separate definition for circular argument because some people like to make a distinction between them. However, the fallacies are actually equivalent in form. Note that they are also sometimes known as petitio principii.
Begging the question:
Premise 1: P.
Conclusion 1: Therefore, P.
The argument is valid because conclusion 1 does follow from premise 1. However, since the intent is to prove P and the argument fails, it is fallacious. The argument is effectively nothing more than a conclusion by itself.
Premise 1: If P, then Q.
Premise 2: If Q, then P.
Premise 3: P.
Conclusion 1: Therefore, Q.
Conclusion 2: Therefore, P.
Notice that premise 3 and conclusion 2 are identical. The argument assumes premise 3 to ultimately prove it as the conclusion, which makes it just a long-winded case of begging the question. The rest of the argument–premises 1 and 2 and conclusion 1–is unnecessary, but might convey a convincing sense of complexity.
What seems to distinguish this argument from general question begging is this logical loop: P->Q, Q->P. However, since that loop is merely a set of two conditionals, it does not constitute an argument. At some point in a circular argument, one assumes P to prove Q to prove P. In this sense, a circular argument is not unique; its defining characteristic is actually extraneous to the fallacy of begging the question.
Clever rhetoricians know that effectively disguising fallacious but nevertheless valid arguments can be as simple as playing with language or even saying a lot. For example, in the form of begging the question provided above, they might disguise conclusion 1 as a synonym for P. Or, in the form of circular argument provided above, they might add many more premises and other conclusions, creating the illusion of a complex argument and burying the only parts that really matter in a massive text or speech.
In an interrogation of some sort, a clever interrogator might ask a question like the famous “When did you stop beating your wife?” This question assumes that the accused man did beat his wife. However, the interrogator wants to either make him answer reflexively, which confirms the assumption that he is a criminal, or confuse him, which could make him stumble and seem guilty or lead to a mistake in the telling of a dishonest story. Moreover, if the questioning takes place in front of an audience, repeatedly employing this fallacy can actually reinforce the idea in the audience’s mind that the accused truly is guilty.
When the fallacy is not employed to deceive, it is often for the sake of answering an otherwise unanswerable question. Consider:
Question: Why are the physical laws of the universe the way they are?
Answer: Because that is the way they are.
Here, the fallacy takes the place of “I don’t know.” Unfortunately, it does not always carry the same humility, since the answerer may be implying that the question is unimportant, perhaps as a defense of his own ignorance.
1.) The Bible is inerrant because the Christian God inspired it. It says that the Christian God exists. Therefore, the Christian God exists. Therefore, the Bible is inerrant.
2.) I like spicy food because it is good. (After all, “is good” is an expression of “I like.”)
3.) Owls are nocturnal because they are active at night.
4.) I think, therefore I am. (“I am” is implicit in “I think.” To be fair to Descartes, however, his argument was a bit more complex. He believed that thought uniquely implies existence because it cannot be faked, whereas our senses could be fooled by greater, supernatural powers. Only something that exists can think; therefore, where there is thought, something real must exist. Descartes could only know with certainty that he could think, not that others could; therefore, he could only conclude that he really existed.)
See the section on Usage. The effectiveness of these fallacious arguments usually depends on the rhetorical skills of those who employ them. However, since those arguments’ core form–P, therefore P–is so obviously silly to even those who are not trained in formal logic, anyone can avoid being fooled by carefully laying out the arguments he hears and ignoring the irrelevant premises.