Big Brother is watching you. So are little sister, the neighbors, your boss, and mom and pop. And it's more and more likely that you're watching them too.

It is precisely this mass participation in surveillance-along with the difficulty of assigning sole responsibility to insidious corporate and government entities-that makes this story so compelling. Even more compelling-and sobering-is that we are losing our privacy not for some nefarious purpose but for the best of reasons: safety, security, and generally improved quality of life. Indeed, surveillance cameras in public spaces, parking garages, and around our neighborhoods can deter crime and speed its solution. Cameras keep tabs even on the police themselves-helping avert the next Rodney King incident. Video cameras, linked to the Web, let us check traffic, and they offer the peace of mind that comes from seeing our kids having fun at home or preschool while we're at work. And video cameras are just the most conspicuous component of the rapidly propagating surveillance infrastructure: at more and more schools, Internet usage is monitored to ensure students don't visit pornography sites; similar systems watch over employees to guard against illicit actions and possibly against harassment, sexual or otherwise. These and other kinds of monitoring can make our lives easier and safer.

And that, ironically, is what makes ubiquitous surveillance so dangerous. We have put ourselves in a position where even though the motivations are good, the outcome could be chillingly bad. Not only governments, but also private groups and individuals, have real opportunities to amass an unprecedented amount of information about each of us-the people with whom we have associated, what we have bought, where we've been, maybe even where we are. Such data, gathered for seemingly benign reasons, could nonetheless be used for suppression of free speech, idea control, and (shades of the movie Minority Report) to finger people for some future crime-purposes that rumble the foundations of free societies.