the.com/circular reasoning

means A logical error where the conclusion of an argument is used as one of its own premises, so the argument just loops back on itself instead of proving anything.

from The word 'circular' traces back to Latin circulus, 'small ring,' from circus, 'ring' or 'circle' — the same root that gives us circus and circuit. Logicians borrowed the shape itself as metaphor: an argument that should move forward from premise to conclusion instead bends back and meets its own starting point, like a path that returns to where it began without ever going anywhere. The term 'circular reasoning' (and its cousin 'circulus in probando,' the circle in proving) has been used in logic for centuries to name this exact failure — not a single coinage by one person, but a natural fit of image to flaw that logicians settled on and kept.