In 1964, Supreme Court Justice Potter Stewart described pornography as something that defied precise definition, “but I know it when I see it.” This week, Justice Stewart’s successors took up again the question of what’s dirty and what’s not and concluded that you often can’t know what you’re seeing.

Of course, the Supreme Court of 38 years ago only had to consider racy novels, grainy movies, the kind of magazines that came in plain brown wrappers and the occasional lounge act gone too far. Today’s court has the Internet, image-altering digital technology and the advanced skills of Hollywood make-up artists to contend with.

The case before the court was a challenge brought by the Free Speech Coalition to portions of the Child Pornography Prevention Act of 1996 in which Congress attempted to ban images of sexual activity involving children. By a 6-to-3 vote, the court ruled that the restrictions on such images were so vague as to be unconstitutional.

It was a decision that spanned the court’s ideological spectrum, from moderate conservative to conservative, with Justices Kennedy, Stevens, Souter, Ginsburg, Breyer, Thomas and O’Connor – in three separate opinions – in general agreement that CPPA does not sufficiently distinguish between real children engaged in real sexual activity and virtual images, most often computer-enhanced trick-the-eye manipulations of legal-age participants, that produce what appears to be kiddie porn without actually exploiting children. The Justice Department’s primary argument in defense of the law, that virtual child pornography could fuel the demand for the real thing or promote pedophilia, was, the majority ruled, contingent and indirect.

Further, the majority held, CPPA could have a chilling effect upon films of serious artistic value that explore youthful sexuality – such recent award-winning films as “Traffic” and “American Beauty” – but that use legal-age actors to play underage roles.

One important provision of CPPA does remain intact after the court decision – the ban on “morphing,” in which a pornographer takes a legal photograph of an actual child and manipulates it on a computer to appear the child is engaged in sexual activity. That portion was not challenged by the Free Speech Coalition because it involves a real child and could be considered exploitation.

Though disappointed by the ruling, Attorney General John Ashcroft said he will work with Congress on new legislation that conforms with the court’s position. The ruling also could lead to the reversal of numerous convictions gained under CPPA, including that of David Hilton of Norway, who was sentenced to 40 months in federal prison in 1990 for gathering kiddie porn computer images for what he unsuccessfully argued was an anti-pornography crusade.

This ruling, though hardly startling in the context of the First Amendment, does jolt would-be Internet censors: The government can regulate obscene material, but it cannot infringe upon protected speech, even when the speech is not what it appears to be.

The next case before the court in this same digital-age vein may be a challenge to the Children’s Internet Protection Act, which withholds federal technology funds from schools and public libraries that refuse to block obscene Web sites. In 1973, in the Miller case, the court ruled that obscenity could be defined by the standards of the particular community in question. With the entire planet now brought together into one Internet village, today’s court should have quite a time defining the standards of a community with no boundaries.