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Full Idea
'Petitio principii' is when the conclusion to be proved is disguised in other words, and put for the definition or principle from whence it is to be demonstrated.
Gist of Idea
'Petitio principii' is reusing the idea to be defined, in disguised words
Source
Thomas Hobbes (De Corpore (Elements, First Section) [1655], 1.6.18)
Book Ref
Hobbes,Thomas: 'Metaphysical Writings', ed/tr. Calkins,Mary Whiton [Open Court 1905], p.39