Certain categories of speech, well-recognized at the personal, opinionated level, have no official existence as far as the free speech rulebook is concerned. Pornography is one such category that has no legal content; we are each entirely free to tar as pornography any sexually explicit expression we dislike, while characterizing as erotica or even romance any of which we approve. Pornography is speech protected by the First Amendment unless it falls afoul of the definition of "obscenity." Pornography is pretty commonly understood to be a category, like "art" itself, based on subjective human reactions and therefore not subject to any precise definition. Susan Sontag, in her essay The Pornographic Imagination, establishes pretty conclusively that the line between pornography and art is impossible to draw.
Justice Potter Stewart, who did not mean to be funny, said he could not define obscenity but "I know it when I see it." The Supreme Court flip-flopped for decades, continually redefining obscenity, until, in 1973, it settled on the so-called Miller standard, according to which obscenity is the subset of pornography which is prurient, patently offensive, and lacking in significant scientific, literary, artistic, or political ("SLAP") value. Therefore, obscenity is defined as a subset of something which cannot itself be defined. This definition, constantly in use in the more than twenty years since Miller, betrays a weakness in the rulebook, or rather, in the Supreme Court's implementation of the American version of the rulebook.
A good Supreme Court rule is internally consistent, logical, and fair. The Miller rule is none of these. It is a machine which gets its work done in a way that doesn't make any sense.
Miller, like the case law before it, presupposes that obscenity is not speech and therefore not protected by the First Amendment. Note that this was not a necessary step to ban or regulate pornography; the courts could just as easily have determined that pornography was less-valued speech. By holding it not to be speech at all, the courts laid the ground for Miller's most obvious illogicality. Miller defines expression as "not-speech" if it is prurient and patently offensive, then throws logic to the winds with the SLAP standard. Since anything with SLAP value is, by definition, First Amendment protected speech, the Miller standard really states as follows:
"Anything which is prurient, patently offensive, and not speech is not speech."
Or you might try to save the standard by phrasing it like this:
"Anything which is prurient and patently offensive is not speech, unless it has SLAP value, in which case it is speech."
Even this definition would be somewhat like defining a bird as "any animal with wings except insects or mammals". It completely fails to define obscenity in terms of what it is, rather than what it is not.
Procedurally, the material being evaluated for obscenity becomes a sort of football. The prosecutor, claiming it is prurient and patently offensive, throws it out of bounds. The defense attorney, showing it has SLAP value, throws it back in. Since under our system, everyone is innocent until proven guilty, placing the burden on the defendant to show SLAP value denies due process of law. Proving the accused material to be "not speech" is a key element of the crime. The onus should therefore be on the prosecutor to prove there is no SLAP value, not on the defendant to show that there is.
Another gross defect in the Miller standard is that obscenity, rather than being defined in terms of some objective, internal qualities, is defined entirely in terms of how we react to it. We are otherwise left clueless as to what it is. Prurience has been paraphrased as meaning that the material "turns us on", while patent offensiveness means that it also "grosses us out". Of course, how I feel about something may tell you nothing about it. A humorous example is the scene in Woody Allen's Manhattan where he states that two men are anti-Semites because he overheard one saying to the other "Jew eat?" and the other reply, "No, Jew?" The comment gives us insight into Allen's personage, but none whatever into the intentions of the men he overheard. Obscenity law means that someone--the speaker of the illicit speech--can go to prison for the listener's reaction or even over-reaction, largely divorced from any actual intention of the speech itself. In a recent case, a student was convicted of ordering a videotape which portrayed clothed adolescent girls dancing. Since prurience and patent offensiveness are determined by the local standards of the community in which the prosecution takes place, nothing in Miller legally or logically prevents a prosecutor from charging, or a jury from finding, that the kissing scenes in Gone With the Wind are obscene.
Prosecutors have run into unusual difficulties in cases where the behavior being described is so kinky that jurors do not find it a turn-on and therefore cannot satisfy prong one of Miller. For example, shoe fetishism is something to which the average juror cannot relate. Therefore, the prosecutor calls an expert witness to explain to the jury that someone--the intended listener-- finds the speech in question prurient. This raises the insane paradox that speech can be held obscene if someone other than the jurors finds it prurient, but can never be found legal if someone other than the jurors finds it not to be prurient. In other words, we will bring in the standards of others to persuade the jury to convict, but never to allow them to acquit (though the defense is permitted, sometimes, to introduce experts to convince the jury that their own standards are other than what they think they are.)
The free speech rule-book, as embodied in our First Amendment, avoids any content-based regulation of speech. If you are allowed to burn a flag or a cross, you are allowed to do so for all purposes. We don't have laws saying that you can burn a flag to express something else but not to criticise our country. The basic tenet of the rule-book, "there is no such thing as an evil idea," means that we will not inquire into the contents of speech. Obscenity laws flagrantly contradict this rule, making our implementation of the rule-book flawed. Why is sexual speech considered "not speech", while violent speech is protected? The contemporary form of cinema I call the gun play is prurient in its own way, appealing to our violent impulses and even (in a lightly disguised way) to our sexuality. It is also patently offensive by my standards. Why is it presumed to have SLAP value, while sexual speech is presumed not to unless proven otherwise? Some inflammatory political speech also panders to base instincts and is patently offensive. The Turner Diaries, in which people exactly like me are strung up from trees, might be termed political "obscenity", but is universally considered to be First Amendment protected. The American implementation of the free speech rule-book adapts the basic tenet to say "There is no such thing as an evil idea, unless it is sexual."
In software terms, Miller was a kluge, a bad hack. (Its application of community standards, which I have dealt with elsewhere, is also senseless in an era of global networks and mass media.) An honest implementation of the free speech rulebook would lead us to the inescapable conclusion that obscenity, like pornography, cannot be defined. Then we could turn our attention to the only issue that makes sense: how to make sure that our children do not see material which, in our personal opinion, is hurtful for them. When the sole consequence of a community standard is that parents and communities can decide what speech to listen to, and no-one goes to prison as a result, the rulebook will be functioning in its full glory.