petitio principii

From Latin petitio principii (literally “an assumption from the beginning”), calque of Ancient Greek τὸ ἐν ἀρχῇ αἰτεῖσθαι (tò en arkhêi aiteîsthai, “to assume from the beginning”).

petitio principii

1. (philosophy, logic, uncountable) The logical fallacy of begging the question.
2. (philosophy, logic, countable) A particular argument which commits the fallacy of begging the question; a circular argument.
   • 1869, C. S. Pierce, "Grounds of Validity of the laws of Logic: Further Consequences of Four Incapacities." Journal of Speculative Philosophy.

     A somewhat similar objection has been made by Locke and others, to the effect that the ordinary demonstrative syllogism is a petitio principii.

   • 1938, E. Prokosch, A Comparative Germanic Grammar.

     The Streitberg-Michels Theory is evidently a petitio principii. To explain ē in gēbum, it is from the outset taken for granted, for inadequate reasons of method, that the form must be a perfect. [italics original]