To understand what Edward Snowden did — out a massive National Security Agency surveillance program that he says is undermining American democracy — it's helpful to go back to Sept. 11, 2001.

Remember the anger and defiance you felt that day? The fear? I certainly remember — like it was yesterday. I watched the towers burn and crumple from my Manhattan office building. After the shock wore off, this troubling question lingered on: "Why couldn't we have done more to stop it?

The answer was, sadly, that we could have done more to stop 9/11. We could have paid attention to 954-815-3004, a telephone number used by Mohamed Atta, who crashed American Airlines Flight 11 into the North Tower. We could have paid attention to phone calls made by Nawaf Al-Hazmi and Khalid Al-Midhar, whose home number was brazenly listed in the San Diego phone book — before they became two of the hijackers on American Flight 77, which slammed into the Pentagon.

Then there were the calls made between the hijackers and contacts in Germany and the Middle East. There were credit card transactions, driver's license records, and more — a string of clues that, if stitched together ahead of time, might have prevented the morning that shattered our complacency about safety and privacy. We've been debating both ever since.

It's also why we've had a global electronic dragnet ever since. Is this intrusive, as Snowden claims? That's in the eye of the beholder. The government knows what numbers are being called by my number, what numbers are calling me. But it doesn't know it's me calling my Mom, or her calling me. No names, just numbers calling numbers. If the government was actually listening, that would be different. But it needs reason to do so, and then a court order.

This system has stopped plots. The government cites the 2009 arrest of Najibullah Zazi, the Afghan-American who was picked up before he could bomb the New York subway. How was he nabbed? The government had reason, based on his communication patterns, to actually listen in. Next time you ride the New York subway, think about that.

But how much security is enough — and at what cost to the privacy we Americans regard as our birthright? President Obama captured the dilemma well on Friday:

I think it's important to recognize that you can't have 100 percent security and also then have 100 percent privacy and zero inconvenience. We're going to have to make some choices as a society.

That's a reasonable observation. The delicate balance between vital yet seemingly conflicting demands — privacy and security — that's the real debate here. It's an issue that eclipses partisan politics. How do we as Americans want to live?

Obama himself has suggested the difficulty in finding that balance. In a major national security speech three weeks ago, he said homegrown terrorism (like the Boston marathon bombing)

presents particular challenges in part because of our proud commitment to civil liberties for all who call America home. That's why, in the years to come, we will have to keep working hard to strike the appropriate balance between our need for security and preserving those freedoms that make us who we are. That means reviewing the authorities of law enforcement, so we can intercept new types of communication, but also build in privacy protections to prevent abuse.

The president (who is also a constitutional law professor) then acknowledged the need to do more on the civil liberty side of the equation. America must, he said, put "careful constraints on the tools the government uses to protect sensitive information, such as the state secrets doctrine. And that means finally having a strong Privacy and Civil Liberties Board to review those issues where our counterterrorism efforts and our values may come into tension."

That's all well and good, but when Obama says "balance," he doesn't mean 50-50. As was the case for Lincoln during the Civil War (who suspended civil rights and shut down scores of newspapers in the name of security) and George W. Bush in the wake of 9/11, presidents tip the scales in favor of what they think will best protect the American people.

Then there are those like Snowden who argue that upholding our civil liberties is the best way to protect the American people, that the corrosive effect of surveillance is just as much a threat as those who wish to physically attack us. That is a legitimate debate. Snowden says the government is going too far; perhaps it is. But let's hear his ideas on maintaining that balance. The question again: How much security and surveillance is enough? And how much is too much?