Begging the question

History

The term was translated into English from the Latin in the 16th century. The Latin version, Petitio Principii (petitio: petition, request; principii, genitive of principium: beginning, basis, premise of an argument), literally means "a request for the beginning or premise." That is, the premise depends on the truth of the very matter in question.

The Latin phrase comes from the Greek en archei aiteisthai in Aristotle's Prior Analytics II xvi:

"Begging or assuming the point at issue consists (to take the expression in its widest sense) in failing to demonstrate the required proposition. But there are several other ways in which this may happen; for example, if the argument has not taken syllogistic form at all, he may argue from premises which are less known or equally unknown, or he may establish the antecedent by means of its consequents; for demonstration proceeds from what is more certain and is prior. Now begging the question is none of these. [...] If, however, the relation of B to C is such that they are identical, or that they are clearly convertible, or that one applies to the other, then he is begging the point at issue.... [B]egging the question is proving what is not self-evident by means of itself...either because predicates which are identical belong to the same subject, or because the same predicate belongs to subjects which are identical."
Fowler's Deductive Logic (1887) argues that the Latin origin is more properly Petitio Quæsiti which is literally "begging the question" as opposed to "petitioning the premise".

An example

"That begs the question" is an apt reply when a circular argument is used within one Syllogism. That is, when the deduction contains a proposition that assumes the very thing the argument aims to prove; in essence, the proposition is used to prove itself, a tactic which in its simplest form is not very persuasive. For example here is an attempt to prove that Paul is telling the truth:

Premise: Paul does not lie when he speaks.
Premise: Paul speaks.
Conclusion: Paul is speaking the truth.
These statements are logical, but they do nothing to convince one of the truthfulness of the speaker. The problem is that in seeking to prove Paul's truthfulness, the speaker asks his audience to assume that Paul is telling the truth, so this actually proves "If Paul is not lying, then Paul is telling the truth." which is nothing more than a tautology.

It is important to note that such arguments are logically valid. That is, the conclusion does in fact follow from the premises, since it is in some way identical to the premises. All self-circular arguments have this characteristic: that the proposition to be proved is assumed at some point in the argument. This is why begging the question was classified as a Material fallacy rather than a Logical fallacy by Aristotle.

Formally speaking, the simplest form of begging the question follows the following structure. For some proposition p:

p implies p
suppose p
therefore, p.
However, the following structure is more common:

p implies q
q implies r
r implies p
suppose p
therefore, q
therefore, r
therefore, p.
The syntactic presentation of the fallacy is rarely this transparent, as is shown for example in the above argument purportedly proving Paul is telling the truth.