Begging the Question
Assuming the truth of the very thing you're trying to prove — circular reasoning.
Origin
A clumsy English translation of Aristotle's Greek term petitio principii via the Latin. The fallacy is when a premise of your argument is the same as (or depends on) the conclusion. 'Murder is wrong because killing people is immoral' — the premise just restates the conclusion in different words. The phrase is famous for being routinely misused in modern English to mean 'raises the question,' which drives logicians crazy.
Modern usage
Almost always misused. 'Which begs the question, why?' has become the standard journalistic way to introduce a follow-up question — strictly speaking that should be 'raises the question.' Pedants will correct it; almost nobody else will care. The technical meaning still lives in philosophy classrooms and editorial style guides.
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