The specter of pornography is once again haunting the minds of certain American conservatives. "Let's ban porn," Ross Douthat of The New York Times declared last year. Porn is "just a product," he wrote, "something made and distributed and sold, and therefore subject to regulation and restriction if we so desire." In The Daily Caller Zak Slayback recently doubled down on Douthat. "Unlike a Scorsese film or a newspaper, both of which are consumed for artistic enjoyment," he wrote, "pornography is consumed with one outside end in mind: orgasm and masturbation. Its primary purpose is not entertainment (in any colloquial sense of the word) or enlightenment. In that sense, pornography is just a tool, like any other product, and can be regulated like any other product." Slayback added: "Don't let the pornography lobby and libertarians frame this as a free speech issue. It's isn't."

But of course this is a free speech issue, as the U.S. Supreme Court has repeatedly recognized, even in cases in which the Court allowed certain restrictions on "obscene" material to stand.

For more than half a century, the Supreme Court has drawn a line between pornography, which enjoys the protections of the First Amendment, and obscenity, which does not. The distinction between the two shows why any sort of all-encompassing government ban on porn would run afoul of the Constitution.

In A Book Named "John Cleland's Memoirs of a Woman of Pleasure" v. Attorney General of Massachusetts (1966), the Supreme Court considered a state effort to have a sexually explicit book declared obscene and therefore banned under state law. Not so fast, the justices told the Bay State. Even a "patently offensive" pornographic work, the Court held, is still protected by the First Amendment. "A book cannot be proscribed unless it is found to be utterly without redeeming social value."

Seven years later, the Court revised its obscenity test in a landmark ruling that continues to hold sway in all legal debates over porn and censorship. According to Miller v. California (1973), "state statutes designed to regulate obscene materials must be carefully limited." A state may only prohibit a work for being obscene if the work meets all of the following criteria:

(a) whether "the average person, applying contemporary community standards" would find that the work, taken as a whole, appeals to the prurient interest…(b) whether the work depicts or describes, in a patently offensive way, sexual conduct specifically defined by the applicable state law; and (c) whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.

The Court applied the Miller test a year later in Jenkins v. Georgia (1974), a case which centered on the obscenity conviction of a movie theater manager who screened the film Carnal Knowledge, which featured nudity but would hardly qualify as pornographic by today's standards. Writing for the majority, Justice William Rehnquist—nobody's idea of a liberal squish—struck down the conviction.

Although the Georgia Supreme Court had upheld the theater manager's obscenity bust on the grounds that "the First Amendment does not protect the commercial exhibition of 'hard core' pornography," as Rehnquist put it, he took a different view—literally. "Our own viewing of the film," Rehnquist wrote, "satisfies us that 'Carnal Knowledge' could not be found under the Miller standards to depict sexual conduct in a patently offensive way." One can only imagine the sort of conversations that Rehnquist and his colleagues had in chambers that day.

As these cases illustrate, the question of whether sexually charged material rises to the level of potentially prohibitable obscenity turns in significant part on the specific contents of the work at issue. If the government wants to ban the sale or distribution of a particular dirty movie, in other words, the government must be able to convince the courts that the movie "taken as a whole, lacks serious literary, artistic, political, or scientific value." Otherwise, it is protected by the First Amendment.

A one-size-fits-all government ban on pornography, by contrast, necessarily avoids all such case-by-case determinations and thus violates both the Supreme Court's precedents and the broader First Amendment principles they endorsed.

Like it or not, the debate about porn is always a debate about free speech. The hands of every would-be government censor are still bound by the First Amendment.