circular reasoning = a logical fallacy or error in reasoning when the conclusion of an argument is assumed in one of the premises
Lilly: That's a circular argument.†
(source) Meg Cabot, The Princess Diaries, 2000
That sort of circular reasoning-the kind that left the person being grilled hung out to dry-was exactly what you aimed for in a courtroom.†
(source) Jodi Picoult, Nineteen Minutes, 2007
"That's a circular argument," I pointed out.†
(source) Rick Yancey, The Infinite Sea, 2014
