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 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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