Most of us know the six categories of the movie-rating system, and think we know what the system is for. Yet there's an immediate disparity if you think of that other story-telling form - books - where readers and volumes muddle on together, where we take our chances in the dark, as it were, without the help of an outside arbiter to tell us whether they're too violent or too dirty or inappropriate or profane.
But movies are different. At this moment, "House of Wax" and "Kingdom of Heaven" are showing under an "R" rating: No one under 17 can see them unless in the company of a parent or an adult guardian. Theaters more or less try to enforce that code, not because it is the law of the land but because it's part of the movie business.
Few of us remember that that's how the ratings system came to be. Fearful that they would be prosecuted or their films banned, studios elected to regulate themselves through the Motion Picture Association of America - not in the public interest so much as in their own interest.
What does an "R" rating really suggest? First, that just about anyone over 17 can be a legitimate "guardian." Any child of any age or sensitivity who can find an adult outside a theater can see the likes of "The Silence of the Lambs," "Psycho," "Saving Private Ryan," "Kill Bill" or "Reservoir Dogs." I'm not attacking those films. Even the poorest of them is entertaining; the best are ... pretty good. But not even if he were surrounded by family, responsible friends, favorite teachers, or a company of saints would I want my 10-year-old son to see "The Silence of the Lambs." Company at the movies is no great protection. The light finds the lone imagination in the dark, and pierces it. It can cause nightmares and dread.
Yet in the eyes of many theater managers, it's entirely acceptable to have a 3-year-old watch Dr. Lecter eat his victims, so long as his seat and the seat next to it are paid for. In the end, this really is the role of the "R" rating: to keep seats filled when people cannot afford (or be bothered) to hire a proper baby sitter.
In most of Europe, this isn't tolerated. Children are barred from seeing troubling films. Their liberty is restricted - just as when they're ordered to be educated, not to drive, not to drink, not to take drugs. It's a welcome admission from society that children have imaginations subject to damage.
Yet Americans aren't willing to impose such a restriction; the ratings depend entirely on the responsibility of a child's parent, baby sitter, or some other available adult. This is different from the way we treat books. Essentially, we've agreed that books wait to be discovered. And if some unsuspecting infant (of whatever age) finds himself reading something shocking, then he'll throw it down, or fall asleep.
Of course, this ratings system is a game. Among filmmakers it's common knowledge that members of the ratings board can't always remember what they saw the last time. So if a filmmaker tells them he's gone back and changed the scene, the board may accept it. Also, if a filmmaker knows he's got a battle coming, he can put eight questionable scenes in, four of which are deliberate sacrificial victims in the ensuing negotiation with the board. That strategy may save the four scenes the filmmaker really wanted.
Several years ago the ratings board introduced the "NC-17" rating, in part because of all the compromising attached to the "R." The "NC-17" - like "X" before it - was meant to keep anyone under 17 out. By implication, any film with such a rating came to be considered "filthy," or worse. And that's the final damning of the ratings system.
A civilized society ought to be able to have "NC-17" material accessible only to grown-up minds. Granted, being 17 is no assurance of a grown-up mind. But "NC-17" is now taboo: It restricts the places where a perfectly good film can play. That means: In the greatest and most enlightened of nations there is effectively no place at the movies (an art or a business that many see as an American invention) for material, talk, and scenes requiring adult experience and understanding.
We should guard our young more carefully, if only because that may permit a proper terrain for adults.
• David Thomson is the author of 'The Whole Equation: A History of Hollywood.' © Los Angeles Times.