Sentence:  The Streitberg-Michels Theory is evidently a <WSD>petitio principii</WSD>. 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] 
Meaning of the word marked with <WSD> tags:  A particular argument which commits the fallacy of begging the question; a circular argument.