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Related to petitio principii: ignoratio elenchi
pe·ti·ti·o prin·ci·pi·i(pə-tĭsh′ē-ō′ prĭn-sĭp′ē-ē′, -ē-ī′)
The fallacy of assuming in the premise of an argument that which one wishes to prove in the conclusion; a begging of the question.
[Medieval Latin petītiō prīncipiī : Latin petītiō, request + Latin prīncipiī, genitive of prīncipium, beginning.]
petitio principii(pɪˈtɪʃɪˌəʊ prɪnˈkɪpɪˌaɪ)
(Logic) logic a form of fallacious reasoning in which the conclusion has been assumed in the premises; begging the question. Sometimes shortened to: petitio
[C16: Latin, translation of Greek to en arkhei aiteisthai an assumption at the beginning]
pe•ti•ti•o prin•ci•pi•i(pɪˈtɪʃ iˌoʊ prɪnˈsɪp iˌaɪ; Lat. pɛˈti tɪˌoʊ prɪŋˈkɪp ɪˌi)
a fallacy in reasoning resulting from the assumption of that which in the beginning was set forth to be proved.
[1525–35; < Medieval Latin petītiō prīncipiī, translation of Greek tò en archêi aiteîsthai the assumption at the outset]