The encounter, if one can even label it as such, lasted no more than a second or two, but in that fleeting moment I recognized the gray-haired man as a plainclothes policeman of some sort, a detective from Scotland Yard, perhaps, or maybe an agent from MI5. And just as I recognized him for what he was, I'm confident he recognized me as an American policeman.

Yes, we had profiled each other. We each had used our experience and powers of observation to form an instantaneous opinion about the other. In that span of a second or two I had noticed him, sized him up, and recognized what he was doing there in the train station, all while he was doing the same to me. My wife and I went on our way, and the gray-haired man remained in his spot watching the arriving passengers for anyone who might invite further scrutiny.

When profiling is done properly, by trained and experienced men and women, it is most often no more intrusive than my encounter with that man in the train station. I doubt any of my fellow passengers gave the man so much as a thought as they passed him, even as he was forming his opinions about every single one of them. But occasionally there might be someone, besides the occasional vacationing policeman, who might be aware of the gray-haired man's attention and indeed be discomfited by it a good deal more than I was that day at Waterloo Station: a drug smuggler, perhaps, a wanted criminal, or even a terrorist.

“Profiling” is such a charged word these days, so freighted with images of people being singled out and inconvenienced at best or detained and even jailed at worst for no other reason than their membership in this or that minority group. But profiling is nonetheless a legitimate and effective technique for identifying people more likely than others to be involved in criminal activity or terrorism, and with the manifest failure of various government entities to prevent Umar Farouk Abdulmutallab and his incendiary skivvies from boarding a U.S.-bound airliner on Christmas Day, it is a technique we should be less shy about embracing.

On Friday, Allison Kaplan Sommer wrote about the multi-layered methods used to foil terrorist plots against commercial flights at Israel's Ben Gurion International Airport. The most important aspect of these methods is the interview each passenger goes through with people trained to detect evasiveness or subterfuge. Rather than subject every passenger to the inconvenience and indignity of pat-downs and endlessly repeated trips through metal detectors, the Israelis, who have come by hard experience to know a few things about terrorism, concentrate on those passengers more likely than others to pose a security risk to an airliner.

Anyone who has traveled by commercial flight since the attacks of 9/11 has endured the routine: You've allowed yourself plenty of time to arrive at the airport, but you get in the security line and glance at your watch as you wait and wait and wait and try to calculate how long it will take to pass through the checkpoint if things continue at their current excruciatingly lethargic pace. Will you have time to get some coffee or something to eat before boarding? To use the bathroom? And you watch helplessly as old women and young children are screened, scanned, wanded, and patted down, their belongings rifled through in an effort to detect … what? When at last your turn comes, you place your belongings in one of those plastic bins you once found only in restaurants, and you remove your shoes and place them on the conveyer belt to await your turn through the metal detector. Then you think, will my belt buckle set off the alarm? My wristwatch? My dental work? By the grace of God you make it through on your first attempt, and you retrieve your shoes and your carry-on bag and your overcoat, but … where's your boarding pass? You had to show it to some charmless TSA agent only moments ago, but now it's … good Lord, where?

Oh yes, it's right there in your pocket. Whew! So now you can proceed toward your departure gate and perhaps take the time to get some coffee or use the bathroom, but certainly not both.

How much more efficient would it be if every airport checkpoint were staffed with a few people like that gray-haired man at Waterloo Station, people experienced enough to recognize those few passengers coming through who might need to be asked to step aside for some additional questioning and perhaps additional searching. And if the number of young male Muslims chosen for this additional screening happens to be greater than might be achieved through some blindly random selection process, so what?

But no. We must all be inconvenienced so that none of us is offended. Profiling? Heavens, not us! And we are now told we must further entrust our safety to technology in the form of body scanners that might detect the combustible undergarments of any passenger who happens to be chosen to enter one, this despite the knowledge that al-Qaeda terrorists have already used a technique to conceal explosives that would render such body scanners worse than useless.

We change our methods, the enemy changes his. We shouldn't trust machines to keep terrorists off of airplanes. We should trust people like the gray-haired man at Waterloo Station. Umar Farouk Abdulmutallab would never have gotten by him, no matter where he had his bomb hidden.