I realize it seems odd to think about an operating system affecting a constitutional question. But this is just the oddness of law in cyberspace. What the Constitution requires will often turn on what the code enables. And no example makes that point better than Congress' continued efforts to regulate pornography.

The latest is the Child Online Protection Act, which mandates that a site check for an adult ID before it transmits material "harmful to minors." Civil-rights groups have challenged this law, arguing that "voluntary" filters would be a less restrictive alternative to government regulation.

I've taken some heat for arguing the reverse. Censorware spreads far beyond users' machines (to the ISP, or corporation, or in some countries, the government), and it filters much more than porn. A properly crafted law, I've suggested, would block much less speech than filters.

This argument has now gotten stronger in a way I had not originally seen and in a way that torpedoes COPA.

Some background: Machines on the Net identify themselves. When you connect to a server, your machine reports the version of its operating system, as well as the browser being used. Apple's OS 9 has the potential to tune that ID. Its new "multiple users" feature conditions access to files and applications depending on the user. If Mom signs on, then her bookmarks are enabled; if Dad signs on, then Mom's e-mail can't be seen.

So imagine one slight change to this "multiple users" feature - the option to specify that a user is a kid. So configured, the machine then could signal servers that the user wanted G-rated surfing, or gSurfing.

This idea would block porn, of course, only if servers cooperated. And here, then, is the place for a simple law: If a client signals gSurfing, then a server may not transmit material "harmful to minors."

There are plenty of details that would have to be worked out for this solution to fly. (The most obvious is to assure that the tag isn't used to target kids.) But this law, I believe, is (1) less restrictive than COPA, (2) just as effective as COPA and (3) better than filters.

Less restrictive: COPA requires adult IDs. Adult IDs are both expensive to acquire and expensive to process. More fundamentally, they raise glaring privacy and free-speech questions. Why should anyone be required to register before exercising the right to access constitutionally protected speech?

The concept of gSurfing raises none of these concerns. No ID is required, so nothing needs to be verified. There would be some burden on Web sites to modify their code, but this is trivial and far less burdensome than installing an ID system.

Just as effective: Though I've used OS 9 to demonstrate the idea, many in the market can provide the same functionality. Netscape already has "profiles" for its browser, and there's no reason Microsoft (MSFT) couldn't do the same. Of course, a kid could go to a machine that isn't gSurfing enabled. But under COPA, a kid can "borrow" a credit card number to the same end. Neither regime is terribly effective. But the point is that they are equally not terribly effective.

In constitutional lawspeak, the possibility of a less restrictive but just as effective alternative to COPA renders COPA unconstitutional. And so will the Supreme Court eventually find, sending the "problem" of porn back to Congress.

There's nothing surprising about this - just reason to be skeptical of the whole process. Congress has little reason to care about the Constitution. By passing COPA, it gets rewarded for "kid-protective" legislation; it's not punished for unconstitutional legislation. Congress in fact knew about gSurfing when it passed COPA. Professor Mark Lemley and others had pointed it out; Sen. Patrick Leahy presented it on the floor of the Senate. Legislators ignored Leahy and Lemley. The courts deal with the Constitution; Congress deals with re-election.

And likewise with the civil-rights groups. They like the simple answer - no law - even though at least the ACLU and EFF have been quite sensitive to the dangers in private filtering. Yet it's too difficult, I've been told, to tell the complicated truth. "People" won't get it.

But there's a cost to this dance. While the issue is unresolved, censorware spreads. If we really cared about the free flow of protected and controversial speech, we would stop the spread even if it meant acknowledging a complex idea: that in this case, a small regulation would be better than none.

Lawrence Lessig the Berkman Professor of Law at Harvard Law School, is the author of Code and Other Laws of Cyberspace, to be published this month by Basic Books.