ALONG with buckets of money  more than $1.6 billion so far, worldwide  and a couple of Golden Globes, James Cameron’s “Avatar” has collected a smattering of controversy. Some of the hue and cry has involved matters of political allegory and theological implication, as pundits have divined that this globally popular blockbuster may represent a veiled ideological attack on America, capitalism, humanity, monotheism or all of the above. But the fiercest attack on “Avatar” has focused on what may seem, compared to such lofty matters, like a minor detail. Of all the corny lines and ready-made catchphrases in Mr. Cameron’s script, perhaps none has turned out to be so provocative as one uttered by Grace Augustine, the scientist played by Sigourney Weaver: “Where’s my damn cigarette?”

In the view of anti-smoking activists, the correct answer should be: Nowhere, at least not in any real or imaginary world governed by a PG-13 rating. The logic of the Smoke-Free Movies campaign, which seeks an R rating for almost all instances of on-screen puffing, is straightforward enough. If the Motion Picture Association of America’s ratings board advises parents about sex, violence, language and drug use, why should it not also shield children from exposure to a lethal (if legal) product that hooks, sickens and kills hundreds of thousands of people a year? Since 2007 the M.P.A.A. has considered smoking when it makes its judgments, and one studio, Disney, has since then made all its family films smoke free.

Should that be true of all movies likely to be seen by children? Does it matter that Grace’s smoking, according to Mr. Cameron, is meant to emphasize the less attractive aspects of her temperament, including that she “doesn’t care about the human body”? And if that mitigation seems like a bit of a stretch (in the future, how likely is it that scientific laboratories on distant moons will allow what their earthbound counterparts forbid today?), what about some of the other recent instances tarred, as it were, by the opprobrium of Smoke-Free Movies? The principal smoker in the animated “Fantastic Mr. Fox” is a villain, and if the hero in “Sherlock Holmes” takes a draw or two on a pipe, well, he is Sherlock Holmes.

In the movie-smoking debate, even clear positions  that children must be protected from images that might influence their behavior, or that filmmakers should be immune from censorship and interference  tend quickly to be fogged with questions of context and nuance. That is because underneath the public discussion about smoking (or gun violence, or sexual promiscuity, or whatever social problem has seized the momentary spotlight) is another, much more confused discourse: about movies and about the ways they mirror and occlude reality.