Campbell claimed the world’s myths are one story wearing a thousand faces: an ordinary soul is called to adventure, crosses a threshold into the unknown, is tested and transformed, and returns home changed, bearing a gift. Slide the hero around the wheel and watch the single road every hero walks.
Campbell’s monomyth (1949) is a 17-stage cycle grouped into three acts — DEPARTURE (5 stages), INITIATION (6), RETURN (6) — the hero crossing a threshold OUT of the ordinary world and, at the end, back INTO it, transformed. The instrument lays the 17 stages on a wheel and walks them. A fail-loud self-check throws unless the three acts partition exactly (5+6+6 = 17) and the final stage returns to the first — a self-consistent CYCLE, closed.
STRONGLY two-layer: what is verified is only the STRUCTURE (17 stages, 3 acts, a closed loop) — NOT that all myths follow it. The monomyth is a LENS Campbell imposed, and folklorists (e.g. the Aarne–Thompson tradition) show it fits selected hero-tales and is forced onto many others; it describes a recurrent PATTERN, not a law of story. A companion to poietike’s narrative-grammar spheres.