petitio principii
Definition: The fallacy of proving something by using a premiss that is itself in as much need of proof as the conclusion.
Example: Capital punishment is necessary because it reduces the number of homicides.
Etymology: In Latin petitio means "asking or begging."
Oxford English Dictionary: The term's first OED citation is from 1531: "Which kynde of disputyng schole men call Petitio principii, the prouyng of two certaine thynges, eche by the other, and is no prouyng at all."
(Tindale Expos. 1 John v. 1–3 Wks. (1573) 420/1)



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