A communion wafer if you can name a double Oscar-winning film which is still banned on these shores in video form, despite its director's opposition. A Clockwork Orange? Uh uh. Stanley Kubrick has prevented its release here. Instead, try William Friedkin's '73 shocker, a tape unlikely to see the inside of a British VCR in non-bootleg form. The BBFC believe it could harmfully affect any 12-year-old girls who watch it, susceptible as they are at that age to the notion of demonic possession and pustulating complexions.
The Exorcist boasts two of cinema's biggest taboos: showing the church attempting to beat the supernatural, and depicting a young girl's traumatic puberty. Think The Last Temptation Of Christ meets Carrie, with some Lolita-style controversy, and you're there: this film's banned because it's too convincing.
Friedkin's triumph is to present us with a world of normality, of dinner parties and detectives who tell Catholic priests: "Did you know you look like Paul Newman?" Then he steadily ruptures this everyday setting with the chaos seething under the surface. The signs are there from the start. Father Merrin (Von Sydow) unearths devilish artefacts on an archeological dig in northern Iraq. A film director plunges from a window, his head near twisted off before he meets the concrete. A priest is troubled by his mother's death. Even the film-within-a-film that Chris MacNeil is shooting focuses on college strife.
Then the Regan-centred horror erupts. No heart-jolting CG effects pulled from the ILM Box Of Pretty Tricks. No Scream-teen frolic of oh-so-clever set-pieces that jolt nervy, giggling viewers from one "Boo! Scared ya!" to another. No schlock-slapstick to sicken with buckets of gore. Just a slow-burning fear that stealthily descends, twisting normality and harrowing souls.
That's where the genius of Friedkin's legacy lies. Not in the acting, which ranges from the perfunctory to the above average (Jason Miller's priest and the emotionally besieged Burstyn stand out). Nor in the special effects, which any half-decent CGI house could now better. But in the very premise itself, so skilfully drawn from Blatty's novel.
For this is the horror practised by Edgar Allan Poe, not Kevin Williamson. There's no novelty-mask killer slicing up neighbourhood teens. It's a clash between two ancient institutions, fighting for a 12-year-old innocent, men of God versus emissaries of the Devil. And fights don't come any bigger.
The end credits roll. You sit there. And you realise that Friedkin has planted a niggling seed of discomfort in your heart that refuses to escape for hours, days, even weeks.