Ten billion dollars is a generous estimate of the property damage done by last month's terrorist attacks. Since Sept. 11, the U.S. stock market has lost more than a half-trillion dollars in value. The difference between the two numbers is one measure of the new fear in the air - and of being in the air.

Our new fear of flying already has put the kibosh on a lot of pointless business conferences, fruitless sales meetings and stressful vacations. In doing so it has disrupted our economy, which is, apparently, premised on pointless business conferences, fruitless sales meetings and stressful vacations.

The economic consequences are sufficiently grave that President Bush asked all the members of his Cabinet to fly somewhere, anywhere, just to demonstrate to the American public that flying commercial isn't a death sentence.

Permit me to report that it isn't. Last week I flew the sort of cross-country route - Washington to San Francisco - targeted by the terrorists. I rarely enjoy flying, and I often dread it.

But my fear obeys a simple rule: The longer it has been since something bad has happened to a commercial airplane, the less happy I feel about boarding one of them. There's always a small risk that something bad will happen on an airplane, and that risk increases when it is ignored. It is most likely to be ignored after a long stretch of tranquility.

Take early September. It was, in retrospect, a very dangerous time to fly, and yet no one seems to have felt frightened or even wary.

Sit back and relax

The corollary to this rule is that when something bad has just happened, as it just now has, and everyone who works near an airplane is on edge, as they just now are, it is, briefly, safe to sit back and relax.

Which I did. The two cross-country flights were the most pleasant I've had in years. The airports and hotels on either end were empty and hungry for company. In that strange place called Business Travel Land, you couldn't swing a dead cat without hitting a free upgrade.

The airplanes themselves were Meccas of kindness and civility, as the few passengers and flight attendants made an uncommon effort to get to know one another, and one another's baggage. There was a moment right after takeoff, I'll admit, when it was impossible not to imagine men leaping into the aisles with shouts of "Allah Akbar." But once the moment passed, I didn't give my safety a second thought, and I don't think anyone else on board did either. We were too busy being fawned over by flight attendants who were intent on making sure each of us wasn't the one with plans to blow up the plane.

But really, what are we scared of? The Sept. 11 attacks aren't likely to be repeated. The first three hijackings succeeded only because they occurred simultaneously, and were entirely unprecedented. Once the passengers in the fourth plane heard what had become of the first three, they foiled the hijacker's intentions. Anyone efficient and purposeful and evil enough to pull off such an attack now knows that he's better off directing his energies elsewhere.

In the second place, there are many new security measures. In this uneasy interlude, before the armed marshals appear in the cabins and the pilots are hermetically sealed in their cockpits, there's actually a strange disjuncture in airline security. The new security measures that are official don't make you feel any more secure; and the new security measures that make you feel more secure aren't official.

Wash hands before continuing

Nixing curbside check-in, substituting plastic for metal knives on meal trays, upping the intensity with which counter clerks ask who packed whose luggage - none of this will inconvenience terrorists.

These official strictures are a bit like the signs on the walls of restaurant toilets that remind employees to wash their hands after they pee. They're designed not to prevent waiters from leaving the bathroom with pee on their hands but to make the customers think that no waiter would ever do such a thing. But if this is so, why must they be asked? These signs wind up making anyone who thinks about them feeling worse.

It's the unofficial security measures that make you feel safer right now. The flight attendants keep a close eye on passengers' every move, which makes for more attentive service. And the security guards ransack the unchecked baggage of anyone who looks even vaguely Arab - which counts as racial profiling but is nevertheless a relief if you aren't Arab, and maybe even if you are.

Added to that, the mechanics and pilots and air traffic controllers and everyone else responsible for ordinary safety is, at least for now, more scrupulous than they ordinarily would be. If nothing else, the general decline in the number of flights gives everyone a bit more time and space to do his job right.

The new fear of flying illustrates a curious aspect of human nature: To make judgments about an uncertain future, people place too much emphasis on the immediate past. As a result, they don't need a clear and present danger to run scared. All they need is a consensus that something frightening has just happened.

A great deal of what we call fear is imposed on us as a kind of social obligation. Everyone is scared because everyone else is scared. Obeying your reason and ignoring the fear caused by the immediate past seems somehow irresponsible, as if in doing so you might get what you deserve.

At this moment, this curious instinct is as alive and well in the financial markets as it is in commercial airplanes. I wouldn't like to say whether this is a good time to buy stocks. But it's a great time to fly.

Michael Lewis is the author of "Next," "The New New Thing" and "Liar's Poker."

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