Brewer's: Petitio Principii

(A). A begging of the question, or assuming in the premises the question you undertake to prove. Thus, if a person undertook to prove the infallibility of the pope, and were to take for his premises - (1) Jesus Christ promised to keep the apostles and their successors in all the truth; (2) the popes are the regular successors of the apostles, and therefore the popes are infallible - it would be a vicious syllogism from a petitio principii.

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