Circularity (dawr) occurs when the conclusion to be established ultimately presupposes itself. Suppose one witness is trusted solely because another vouches for him, while the second is trusted solely because the first vouches for him; without an independent basis the justification is circular.
Not every use of intellect in assessing a proof has this structure. The relevant question is whether the proposition under examination was already assumed in the premises.
Using a faculty and illicitly assuming the very conclusion that faculty is supposed to demonstrate are not identical acts.