The quality of being circular in shape or form; also, the logical fallacy of using the conclusion as evidence for itself.
From Old French 'circuler' and Latin 'circularis,' from 'circulus' (circle). The suffix '-ity' (from Latin '-itas') converts adjectives into abstract nouns. The term has been used philosophically since medieval times.
Circular reasoning is so named because your argument literally returns to where it started without making progress—Aristotle spotted this trap 2,400 years ago and it's still one of the sneakiest logical errors.
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