IN THE MID-1980s, standing up to hard-liners' complaints that China's newly open door to the West was admitting undesirable influences, Deng Xiaoping remarked that it was only natural for a few flies to enter through open windows.

Almost two decades later, that advice is no less timely as the world - from China to America's libraries - copes with the Internet's rapidly spreading potential for no-holds-barred access to information.

In the United States and elsewhere, one of the most vexing problems with the big open window of the Internet is that it not only allows in the fresh air of wider knowledge, it also gives easy entree to an endless range of pornography and other material inappropriate for minors. Three times since 1996, Congress has tried to come up with laws to protect children - the latest being the Children's Internet Protection Act of 2001, which required schools and libraries to use filters to block undesirable web sites.

But Friday, a federal appeals court ruled that this law - as with earlier congressional efforts to limit Internet access - is unconstitutional, saying that current filtering technology is not finely tuned enough to avoid blocking thousands of legitimate Web pages that have First Amendment protection. The ruling, which affects libraries but not schools, was not the easiest option - as it leaves the matter to the uncertainties of adult supervision - but it was the right one.

For libraries, the problem is that it's not possible to impose an accurate automatic filter on the Internet's limitless and ever-changing flow of information. Those who believe that restricting access to legitimate Web sites is an acceptable price to pay for protecting children should shift the context back overseas - to China's continuing opening to the world.

Internet use is growing faster in China than anyplace else in the world, and, as with most things, the Chinese are apt someday to end up as the world's largest online population. Not surprising for a one-party, heavy-handed state, the Chinese government has sought to reap the Internet's considerable benefits (say, for business information) while blocking access to Western news and political sources.

Of course, one man's political threats are another's freedoms.

So Americans likely would support steps toward greater Internet openness in China, such as the recent move in some Chinese cities to stop blocking - at least temporarily - some major U.S. news organizations' Web sites. Many Americans also would likely hope wider Internet access might help topple the Chinese Communist Party. . As much as that would be a decidedly undesirable side effect from the perspective of the Chinese government, it likely would be considered just fine by many Americans.

China can't have it both ways, you see, can't open its windows without letting in some flies. Neither can America's libraries - even if the side effects aren't always the most desirable.