The reading
The bead. An innocent child is defiled from outside by something monstrous, and two priests pay with their lives to drive it out and hand her back clean.
Engines
- purity/contamination · content · spine · ~ — the defiled-victim shape at its starkest: Regan, a child who did nothing to earn it, is contaminated from without (the body desecrated, the voice and flesh no longer hers), and a costly ritual cleansing — the Roman Ritual, paid for with both priests' lives — expels the contaminant and restores her; she wakes clean, with no memory of the defilement. The bright pole, purity actually restored.
- redemption · content · also-runs · ~ — Father Karras's crisis of faith resolves in a sacrificial coda ("Take me!"): he pulls the demon into himself and throws himself out the window. It is the cleansing's cost, not the spine.
- caretaking/being-needed · content · also-runs · ~ — Chris MacNeil's desperate maternal fight — the mother who exhausts medicine and then the church — drives the search for the cure.
The bundle. The purification bundle: a non-moral defilement imposed on an innocent (purity) lifted by a sacrificial cleansing (redemption's cost) undertaken at a mother's behest (caretaking).
Dual-use read. Purity's counterfeit is the purity spiral / scapegoating — declare a contaminant irreversible, then purge it (the Madison Grant register, counterfeit-catalog). The Exorcist sits firmly on the enabling side: the contaminant is real, the cleansing is real and costly, and the defilement is non-moral — Regan is a victim, not a sinner, which is the guard that separates purity from redemption. The hazard register is faint here; the film is the engine's clean home, the demonic-possession genre's reason to exist.
Verdict. The bright-pole companion to Dracula: purity/contamination delivered positively, the contaminant confronted and expelled at mortal cost and the stainless self handed back — exactly the wish possession-horror is built to service.
Evidence. ~ reviewed — from the film (in-copyright); source novel Blatty 1971. Sibling slot-proven specimen: Dracula / Dracula.