petitio principii

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

• (Received Pronunciation) IPA^(key): /pɪˈtɪʃiˌəʊ prɪnˈkɪpiˌaɪ/
• (General American) enPR: pə-tĭshʹē-ō' prĭn-sĭpʹē-ī', IPA^(key): /pəˈtɪʃiˌoʊ prɪnˈsɪpiˌaɪ/

petitio principii

1. (philosophy, logic, uncountable) The logical fallacy of begging the question (i.e., "assuming the conclusion").
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.”, in 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]