E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 infallibleit would be a vicious syllogism from a petitio principii.