Time magazine recently reported that the Transportation Security Administration, the American agency charged with protecting airplanes, has concluded that the “most dangerous threat to commercial aviation is not so much the things bad people may be carrying, but the bad people themselves.”

Accordingly, Time goes on, the TSA is launching a passenger profiling system known as Screening of Passengers by Observation Techniques, or SPOT. Under SPOT, the TSA staff learns to recognize suspicious personal behavior. “Passengers who flag concerns by exhibiting unusual or anxious behavior will be pointed out to local police, who will then conduct face-to-face interviews to determine whether any threat exists.”

However belatedly, the Bush administration has recognized that terrorists, more than the tools of their trade, must be watched and stopped. This amounts to a gigantic step forward in the protection of American travelers. The administration deserves congratulations for the courage to accept the need for profiling.

But SPOT is just a first step. Skilled terrorists learn how not to be nervous or give off other telltale signs of their plan. To be fully effective, profiling must focus on something more inherent to terrorism than anxiety. What might that be? Here is where the debate gets both productive and interesting.

Michael Smerconish, a radio talk show host and columnist in the Philadelphia Daily News, argues in his new and brave book “Flying Blind: How Political Correctness Continues to Compromise Airline Safety Post 9/11,” that the key factor is race and ethnicity. In contrast, I hold that the key is not external attributes but what is in a person’s head, namely Islamist beliefs.

Mr. Smerconish writes, “We’re fighting a war against young Arab male extremists, and yet our government continues to enforce politically correct, random screening, of airline passengers instead of targeting those who look like terrorists.”

He calls for a change in policy: “Logic dictates that airport security take a longer, harder look at individuals who have ethnic, religious, nationality, and appearance factors in common with the Islamic extremist Middle Eastern men who have initiated war against us.”

This is a step in the right direction, but like SPOT, it is just a start. Yes, young Arab male extremists have carried out most terrorist attacks in the West. Yes, focusing on observable traits like Arabic names or a Middle Eastern appearance is easily done. But, like nervousness, these are crude criteria that do not get to the heart of the problem, which is the Islamist ideology.

A significant number of Islamist terrorists in the West are not Arab or immigrants at all. Their ranks include converts who began life with names like Ryan Anderson, David Belfield, Willie Brigitte, Jerome and David Courtailler, Michael Christian Ganczarski, Clement Rodney Hampton-el, Mark Fidel Kools, Jose Padilla, Adam Pearlman, Richard Reid, Pierre Robert, Jack Roche, and Steven Smyrek. These converts grew up in the West, speak Western languages with no accent, and know the local sports heroes. Some of them are even blond.

Terrorists are not stupid; focusing on Arabs, as Mr. Smerconish urges, will prompt them to turn to non-Arab operatives. This is already a concern. Jean-Louis Bruguiere, the leading French anti-terrorist investigating judge, warned along these lines in May 2003, recounts Robert Leiken, that “Al Qaeda had stepped up its European recruiting efforts and was on the lookout for women and light skinned converts in particular.” The deputy director of a French intelligence agency, the Direction de la Surveillance du Territoire, even told Mr. Leiken that “converts are our most critical work now.”

Mr. Smerconish responds to my argument by acknowledging that his book may at some point become obsolete, “but that day is not today.” He sees physical appearance remaining a key predictor of intentions.

True, young Arab males still play a disproportionate role, but his approach nearly guarantees that will change. Law enforcement should now begin worrying about motives. Islamism, a radical reading of the Islamic religion, prompts Islamist terrorism, not speaking Arabic.

Airport security personnel has found it a challenge merely to catch weapons; finding Arab would-be terrorists will prove more difficult and stopping malign Islamists will be hardest of all, for it requires TSA knowing in some depth who’s who among passengers. But this is the gold standard of counterterrorism and it should, starting immediately, be its goal.

Mr.Pipes (www.DanielPipes.org) is director of the Middle East Forum and author of “Miniatures” (Transaction Publishers).