Pétition de principe

In classical rhetoric and logic, begging the question or assuming the conclusion (Latin: petitio principii) is an informal fallacy that occurs when an argument's premises assume the truth of the conclusion. A question-begging inference is valid, in the sense that the conclusion is as true as the premise, but it is not a valid argument. For example, the statement "wool sweaters are superior to nylon jackets because wool sweaters have higher wool content" begs the question because this statement does not explain why higher wool content makes a garment superior. Begging the question is a type of circular reasoning, and often occurs in an indirect way such that the fallacy's presence is hidden, or at least not easily apparent. The phrase "begs the question" is also commonly used to mean "prompts a question" or "raises a question". The original phrase used by Aristotle from which begging the question descends is: τὸ ἐξ ἀρχῆς (or sometimes ἐν ἀρχῇ) αἰτεῖν, "asking for the initial thing". Aristotle's intended meaning is closely tied to the type of dialectical argument he discusses in his Topics, book VIII: a formalized debate in which the defending party asserts a thesis that the attacking party must attempt to refute by asking yes-or-no questions and deducing some inconsistency between the responses and the original thesis. In this stylized form of debate, the proposition that the answerer undertakes to defend is called "the initial thing" (τὸ ἐξ ἀρχῆς, τὸ ἐν ἀρχῇ) and one of the rules of the debate is that the questioner cannot simply ask for it (that would be trivial and uninteresting). Aristotle discusses this in Sophistical Refutations and in Prior Analytics book II, (64b, 34–65a 9, for circular reasoning see 57b, 18–59b, 1). The stylized dialectical exchanges Aristotle discusses in the Topics included rules for scoring the debate, and one important issue was precisely the matter of asking for the initial thing—which included not just making the actual thesis adopted by the answerer into a question, but also making a question out of a sentence that was too close to that thesis (for example, PA II 16).