Fortunately, in the judgment of this British Airways agent, my passport's mechanical glitch is overridden by this deeper reservoir of personal data. Not only am I traveling with a thirteen-year-old of the same last name, I have not—wittingly—harbored fugitive Al Qaeda cadres from South Waziristan. I'm clean.

The ticket agent pauses before punching out our boarding cards.

"Would you like the exit row?" she asks pleasantly.

All travelers begin with a basic skein of commercial and government data, be it fibrillating lines of credit, mobile phones pinpointing their geographical coordinates in real time, or badly lit photos in a passport. Attended by such a motley nineteenth-, twentieth-, and twenty-first-century data kit, I began in Andalusia at the wedding of some friends. Afterward, I flew to London, Berlin, Milan, St. Moritz, and Venice, then retraced my steps through Milan, Berlin, and London to get home. I used one rented triband cell phone. All told, I carried three currencies, encountered four languages (five, counting Romansh), crossed a total of ten international borders (one border six times; two, four times; three, twice). In doing that, I drove 672 miles and flew some 10,200 miles, stayed in three hotels and one apartment, made 127 mobile phone calls, and ate twenty-eight dinners, fifteen of which I paid for with a credit card. The government entities that took an interest in my identity, or registered my movement, were many: five European border police forces; Interpol; Europol (the transnational police agency in Europe); the TSA; U.S. Customs and, by extension, the FBI, the CIA, and eighteen other U.S. agencies that participate in the Customs watch lists.

The information we generate about ourselves as we move through the world, and how that information is maintained and analyzed, has taken on a decided military tang. The surveillance of civic activity and of civilians—say, whether I regularly buy tea at the Hamburg shop near the mosque patronized by members of Mohammed Atta's 9/11 cell—has become more than just commercially significant. The ability to extract meaning from such a piece of data is now a crucial element of the international community's counterterrorist arsenal. Such a shard of personal information could, if the databases are empowered to find it, keep me off a plane.

The nine satchel bombs placed on Madrid's commuter trains in March were detonated by cell phones. From the one accidentally captured dud, investigators obtained the call list and tracked the perpetrators, a relatively ordinary forensic use of data these days. But now that a two-thousand-dollar Japanese digital toilet can determine its user's glucose levels—presumably without a surprise injection—and e-mail the stats to a doctor, we must reckon with future data probes of the most intimate caliber.

We transit London and land in Málaga. A Spanish border policeman gives my passport a cursory glance, flips to a page in the back, and stamps it with an impressive thud. This is my official entry into the European Union. There's no way he had time to register or check my name. Instead, he has profiled me, old-style. His thinking must run something like this: Bald, bespectacled Caucasian, with family, from fattest target nation on earth—I'm letting him in.