I’m glad I live in a country with people who are vigilant in defending civil liberties. But as I listen to the debate about the disclosure of two government programs designed to track suspected phone and e-mail contacts of terrorists, I do wonder if some of those who unequivocally defend this disclosure are behaving as if 9/11 never happened — that the only thing we have to fear is government intrusion in our lives, not the intrusion of those who gather in secret cells in Yemen, Afghanistan and Pakistan and plot how to topple our tallest buildings or bring down U.S. airliners with bombs planted inside underwear, tennis shoes or computer printers.

Yes, I worry about potential government abuse of privacy from a program designed to prevent another 9/11 — abuse that, so far, does not appear to have happened. But I worry even more about another 9/11. That is, I worry about something that’s already happened once — that was staggeringly costly — and that terrorists aspire to repeat.

I worry about that even more, not because I don’t care about civil liberties, but because what I cherish most about America is our open society, and I believe that if there is one more 9/11 — or worse, an attack involving nuclear material — it could lead to the end of the open society as we know it. If there were another 9/11, I fear that 99 percent of Americans would tell their members of Congress: “Do whatever you need to do to, privacy be damned, just make sure this does not happen again.” That is what I fear most.

That is why I’ll reluctantly, very reluctantly, trade off the government using data mining to look for suspicious patterns in phone numbers called and e-mail addresses — and then have to go to a judge to get a warrant to actually look at the content under guidelines set by Congress — to prevent a day where, out of fear, we give government a license to look at anyone, any e-mail, any phone call, anywhere, anytime.

So I don’t believe that Edward Snowden, the leaker of all this secret material, is some heroic whistle-blower. No, I believe Snowden is someone who needed a whistle-blower. He needed someone to challenge him with the argument that we don’t live in a world any longer where our government can protect its citizens from real, not imagined, threats without using big data — where we still have an edge — under constant judicial review. It’s not ideal. But if one more 9/11-scale attack gets through, the cost to civil liberties will be so much greater.

A hat tip to Andrew Sullivan for linking on his blog to an essay by David Simon, the creator of HBO’s “The Wire.” For me, it cuts right to the core of the issue.

“You would think that the government was listening in to the secrets of 200 million Americans from the reaction and the hyperbole being tossed about,” wrote Simon. “And you would think that rather than a legal court order, which is an inevitable consequence of legislation that we drafted and passed, something illegal had been discovered to the government’s shame. Nope. ... The only thing new here, from a legal standpoint, is the scale on which the F.B.I. and N.S.A. are apparently attempting to cull anti-terrorism leads from that data. ... I know it’s big and scary that the government wants a database of all phone calls. And it’s scary that they’re paying attention to the Internet. And it’s scary that your cellphones have GPS installed. ... The question is not should the resulting data exist. It does. ... The question is more fundamental: Is government accessing the data for the legitimate public safety needs of the society, or are they accessing it in ways that abuse individual liberties and violate personal privacy — and in a manner that is unsupervised. And to that, The Guardian and those who are wailing jeremiads about this pretend-discovery of U.S. big data collection are noticeably silent. We don’t know of any actual abuse.”We do need to be constantly on guard for abuses. But the fact is, added Simon, that for at least the last two presidencies “this kind of data collection has been a baseline logic of an American anti-terrorism effort that is effectively asked to find the needles before they are planted into haystacks, to prevent even such modest, grass-rooted conspiracies as the Boston Marathon bombing before they occur.”To be sure, secret programs, like the virtually unregulated drone attacks, can lead to real excesses that have to be checked. But here is what is also real, Simon concluded:“Those planes really did hit those buildings. And that bomb did indeed blow up at the finish line of the Boston Marathon. And we really are in a continuing, low-intensity, high-risk conflict with a diffuse, committed and ideologically motivated enemy. And, for a moment, just imagine how much bloviating would be wafting across our political spectrum if, in the wake of an incident of domestic terrorism, an American president and his administration had failed to take full advantage of the existing telephonic data to do what is possible to find those needles in the haystacks.”And, I’d add, not just bloviating. Imagine how many real restrictions to our beautiful open society we would tolerate if there were another attack on the scale of 9/11. Pardon me if I blow that whistle.