Petitio Principʹii (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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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.