In the wake of September 11, there was a lively debate about the optimal mix of "hard" versus "soft" power--guns versus diplomacy, military force versus foreign aid. Thursday's foiled plot to blow up commercial jets shows that a similar divide informs the world of police work. Scotland Yard and the FBI sometimes stop terrorists by shooting them, just as the criminal justice system sometimes stops attempted murders by incarcerating the would-be killers. More often, though, violent plans are thwarted by a different sort of policing: light on force and violence, heavy on secrets and lies. Plant informants or bribe sources, figure out ways to listen and watch that the bad guys don't know about. Trick people into telling you their secrets. Get more information. Deceit is the key to that enterprise--all good surveillance and intelligence operations depend on it. No Islamic terrorists want to tell Western intelligence services what their plans are. The only way government agents find out, short of brutalizing suspects or sources, is to trick them into giving the information away. Undercover agents and informants do that by pretending to be friends, colleagues, or even lovers. Effective surveillance programs do it by seeing or hearing things that the targets don't expect to be seen or heard. The idea is for the targets to give away information without realizing it--just like a mark gives his money to a con artist without knowing he's just been conned.