Petitio principii is also known as begging the question, circular reasoning, and circular argument. It is defined as “to beg the question is to assume the truth of what one seeks to prove, in the effort to prove it” (Copi and Cohen, 1990). There are a few kinds of this fallacy. The simplest, most easily seen, and least convincing are using the same words in the same order to both state and prove a proposition.
Person A: “someone that studies medicine is a doctor, because a doctor is someone that studies medicine.”
This is clearly begging the question because person A assumed the truth of the statement (“a doctor is someone that studies medicine”) in trying to prove the statement (“someone that studies medicine is a doctor:”)
An other type of petitio principii is turning premises into conclusions through manipulating language. For example:
Person B: “All parents are loving, for no parents are unloving.”
A third type of this fallacy, which is by far the most difficult type, is where both premises and conclusions have the same propositional content. For example:
J. Wolfe (as cited in “Philosophy 103: Introduction to Logic Petitio Principii.” Internet: http://philosophy.lander.edu/logic/circular.html, [Oct. 21, 2015].): “The elemental composition of Jupiter is known to be similar to the sun… The core would be composed mainly of iron and silicates, the materials that make up most of the earth’s bulk. Such a core is expected for cosmogonic reasons: If Jupiter’s composition is similar to the sun’s, the the planet should contain a small portion of those elements.”
It is important to note that petitio principii (begging the question) is not the same as raising the questions, as Trabbic (2015) points out. Says Trabbic, “‘Begging the question’ is a logical fallacy… on the other hand, “raising the question” is simply bringing a question up for discussion.”
I. M. Copi and C. Cohen. Introduction to Logic. New York: MacMillan Publishing Company, 1990, pp. 126-28.
J. Trabbic. “‘Begging the question’ vs. ‘raising the question.’” Internet: http://philosophy.avemaria.edu/post/29691374480/begging-the-question-vs-raising-the-question, 2015 [Oct. 21, 2015].
“Philosophy 103: Introduction to Logic Petitio Principii.” Internet: http://philosophy.lander.edu/logic/circular.html, [Oct. 21, 2015].