Petitio principii

Definition and stakes

Portrait of John Stuart Mill John Stuart Mill,  A System of Logic, Ratiocinative and Inductive (1881)

“ In any other sense they are clear cases of Petitio Principii, since the word laudable, and the idea of boasting, imply principles of conduct; and practical maxims can only be proved from speculative truths, namely, from the properties of the subject-matter, and can not, therefore, be employed to prove those properties. As well might it be argued that a government is good because we ought to support it, or that there is a God because it is our duty to pray to him. ”
Source: Wikisource

Portrait of G. E. Moore G. E. Moore,  Philosophical Studies

“ So much must be granted to any sceptic who feels inclined to hold it. Any valid argument which can be brought against it must be of the nature of a petitio principii: it must beg the question at issue. How is the sceptic to prove to himself that he does know any external facts? ”
Source: Gutenberg

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