Tomorrow morning, as you're riding your usual bus to work, a passenger stabs the driver, announces that he's wearing a bomb, and orders everyone off the bus. You get off.
You have just facilitated the deaths of 20 children on a sidewalk three blocks ahead of you.
Hours later, people wonder why you and the other passengers allowed the killer to commandeer the bus. You're torturing yourself with the same questions. How could you be so stupid? How could you be so passive?
It wasn't a failure of logic or of will. You simply never entertained the possibility that the man would do what he did. The worst you imagined was that the driver would die and the bus would be stolen.
Remember this scenario when you think of the passengers who were on the planes involved in this week's terrorist attacks. Why did so many of them remain in their seats or in the backs of the planes while the hijackers took aim at buildings containing thousands of people? Because it never seriously occurred to those passengers that the hijackers would use the planes for that purpose. This isn't the first time terrorists have threatened to do such a thing in fact or fiction. But it is the first time everyone has become aware of the possibility.
This is one of the most important lessons of Tuesday's catastrophe. Our ability to prevent crimes is constrained not only by technical limits--money, intelligence assets, airport security administration--but also by conceptual limits. We can't prepare for what we don't anticipate. Furthermore, the range of scenarios we anticipate isn't limited merely by what we think others can do. It's also limited by what we think they will do. Given the countless possible permutations of human behavior, we confine our preparations and calculations to those acts that fall within the range of our implicit moral expectations. We're blind to the risk of unthinkable evil precisely because it's unthinkable.
Terrorists don't have the technical assets that states enjoy. What they have instead is the advantage of surprise, by virtue of their willingness to defy moral expectations. If you can't imagine that they'd target children, they'll target children. If you can't imagine that they'd use your bus to do it, they'll use your bus to do it. If you can't imagine that they'd fly your plane into the World Trade Center, they'll fly your plane into the World Trade Center. Your conscience is their cover.
After a terrorist attack, everything looks different. You realize that every plane is a potential guided missile and that every hijacker might be a kamikaze pilot. Yet nothing has changed except the range of your imagination. You can't blame others for failing to imagine these things, any more than others could blame you tomorrow for failing to realize that the bus you were riding was an excellent weapon for committing multiple homicides.
Today, the airwaves are full of talk about fixing airport security so that terrorists can't commandeer planes so easily again. That's a good first step, but it's not enough. The next terrorist strike won't involve the use of a hijacked plane as a guided missile, precisely because Tuesday's attacks brought that scenario into the realm of moral expectation. Next time, passengers will depressurize or crash the plane rather than let hijackers fly it.
That's what happened Tuesday morning on United Airlines Flight 93. That plane, unlike the other three that were hijacked, went down in an uninhabited area instead of reaching its target. The reason, by all accounts, is that some of the passengers rushed the hijackers. Why did this happen on Flight 93 and not, evidently, on the other planes? Perhaps the people on Flight 93 were particularly courageous. But the most plausible answer is that it was the last of the four flights to be hijacked. Some of its passengers, while speaking to their loved ones on cell phones, were told that planes had been flown into the World Trade Center. The same thing happened on American Airlines Flight 77 a few minutes before it struck the Pentagon. But on Flight 93, the use of planes as guided missiles against densely populated targets became morally thinkable before it was too late.
Where will terrorists strike next? If you've got an answer off the top of your head, it's the wrong answer. To thwart a terrorist, you have to think like one. Imagine the destruction that could be engineered with the help of your car, your credit card, or your computer password. Think of how many people you could kill by hot-wiring a propane truck, breaking into a water tower, or piloting a barge full of unknown cargo into a harbor. The next attack could come by air, land, or sea. All we know for sure is that the route to stopping it begins in our heads.