In his feature debut as a writer-director, Joseph Gordon-Levitt has made a movie about pornography and masturbation. There is more to "Don Jon" than that. It’s daring, has a feeling for its working-class milieu and shows an original comic sensibility. But that exploration — into the function of pornography in the life of one young man, who probably isn’t unique — is 90 percent of the movie’s inspiration and purpose.

That’s how the movie begins, with Jon (Gordon-Levitt), in voice-over, talking about how he becomes aroused just from the sound of his computer turning on. Porn is one of the pillars of his life. He loves it, and masturbation is his favorite form of sex. You might expect, hearing this, that it’s his only form of sex, but as we soon see, he’s something of a ladies’ man. He picks up good-looking women regularly and brings them home for a one-night stand. His friends call him "The Don."

‘DON JON’ Rated: R Opens today

Yet you can see that something is a bit off about The Don when after having sex with an attractive woman he masturbates over porn. What are we to make of this? Is this a harmless predilection, a genuine distortion of the natural impulse, or some kind of abnormal "new normal." Keep in mind, for a young man, the Internet has always been there. Technology has been like a second parent.

"Don Jon" isn’t an issue movie, or rather it doesn’t feel like one. It’s a story about a particular guy in a particular place. Jon is Italian-American and lives in New Jersey. Although the accents are more like New York and the cultural influences (close family, the Catholic Church) are more nostalgic than contemporary, Gordon-Levitt knows what to exaggerate to comic effect. Tony Danza, as Jon’s father, whose temper is always on a hair trigger and who only wants to watch the football game, is particularly dead-on and very funny.

Jon and his two buddies go out every weekend, rating on a 1 to 10 scale — Jon won’t go with anyone below an 8 — and one day he spots a 10 standing by the bar, in the form of Barbara, played by Scarlett Johansson.

"Don Jon" is an excellent showcase for Johansson. It presents her at her most alluring and provides her with a character with lots of nuance and authority. Best of all, she gets to play an East Coast type, getting not just the dialect right but the whole attitude and atmosphere.

Most of the picture deals with Jon and Barbara’s relationship (with porn always hovering in the background).

"Don Jon" deserves praise for wearing its message lightly and yet for daring to present such a lecture in today’s Internet-drenched environment. Gordon-Levitt may be blithe in discussing pornography, but his movie nonetheless asserts that porn is addictive and destructive, that it intrudes on intimacy, and that it short-circuits the capacities for interaction and also, ultimately, for pleasure. That’s a serious subject and a committed viewpoint, handled with wit and intelligence.

Julianne Moore has a nice role as a free spirit who says any outrageous thing that pops into her head. Moore is fun, turbulent and full of feeling — everything you might expect from her. But the movie, in its last 15 minutes, loses something, or perhaps just fails to gain something. Gordon-Levitt can’t quite end his film convincingly. He can only stop it, and so there’s little sense of arrival.

Yet if that’s the worst that can be said for "Don Jon," that’s not so bad. At least, Gordon-Levitt gets it done in 90 minutes, instead of belaboring a false finish for another half-hour.