A recent study by the Breakthrough Institute found only two cases that benefited from the secret warrants made easier by the Patriot Act. The rest, the report concluded, “were broken open due to the combination of well-deployed undercover agents, information from citizen or undercover informants and tips from foreign intelligence agencies.” The two exceptions were the Portland Seven, Oregon Muslims who tried to travel to Afghanistan to fight with the Taliban in 2001, and Najibullah Zazi, a Colorado resident from Afghanistan who pleaded guilty last year to planning a suicide attack in the New York City subways.

Two successes in nearly a decade might be enough to satisfy a fearful public, but it is worth noting that both cases began with old-fashioned tips — the first from a landlord, the second from Pakistani intelligence linking Mr. Zazi to Al Qaeda — and could have been pursued with the law enforcement tools available before 9/11.

The false dichotomy of liberty versus security is accompanied by another myth: that someone else’s rights are always the ones at risk, that I can give up their rights for my safety. It seems a comfortable bargain. The terrorist is covertly monitored, the drug dealer is searched and the upstanding citizen is protected.

But it does not always work that way. The constitutional system of case law and precedent applies rulings on rights universally. So, legally, if a black man in a poor neighborhood can be stopped and frisked with minimal reason, so can a white woman in a rich neighborhood — even if the police tactics differ.

American history is replete with assaults on liberties that first target foreigners, minorities and those on the political margins, then spread toward the mainstream. The 1917 Espionage Act, for example, was used to prosecute American labor leaders and other critics of the government, and the 1798 Alien Enemies Act was revived after Pearl Harbor to intern American citizens of Japanese ancestry. A similar process is taking place now, as the F.B.I. has begun using counterterrorism tools to search, infiltrate and investigate groups of American peace activists and labor leaders in the Midwest.

The Fourth Amendment is weaker than it was 50 years ago, and this should worry everyone. “Uncontrolled search and seizure is one of the first and most effective weapons in the arsenal of every arbitrary government,” Justice Robert H. Jackson, the former chief United States prosecutor at the Nuremberg trials, wrote in 1949. “Among deprivations of rights, none is so effective in cowing a population, crushing the spirit of the individual and putting terror in every heart.”