MY colleague Thomas Friedman’s levelheaded take on the National Security Agency eavesdropping uproar needs no boost from me. His column soared to the top of the “most e-mailed” list and gathered a huge and mostly thoughtful galaxy of reader comments. Judging from the latest opinion polling, it also reflected the prevailing mood of the electorate. It reflected mine. But this is a discussion worth prolonging, with vigilant attention to real dangers answering overblown rhetoric about theoretical ones.

Tom’s important point was that the gravest threat to our civil liberties is not the N.S.A. but another 9/11-scale catastrophe that could leave a panicky public willing to ratchet up the security state, even beyond the war-on-terror excesses that followed the last big attack. Reluctantly, he concludes that a well-regulated program to use technology in defense of liberty — even if it gives us the creeps — is a price we pay to avoid a much higher price, the shutdown of the world’s most open society. Hold onto that qualifier: “well regulated.”

The N.S.A. data-mining is part of something much larger. On many fronts, we are adjusting to life in a surveillance state, relinquishing bits of privacy in exchange for the promise of other rewards. We have a vague feeling of uneasiness about these transactions, but it rarely translates into serious thinking about where we set the limits.

Exhibit A: In last Thursday’s Times Joseph Goldstein reported that local law enforcement agencies, “largely under the radar,” are amassing their own DNA databanks, and they often do not play by the rules laid down for the databases compiled by the F.B.I. and state crime labs. As a society, we have accepted DNA evidence as a reliable tool both for bringing the guilty to justice and for exonerating the wrongly accused. But do we want police agencies to have complete license — say, to sample our DNA surreptitiously, or to collect DNA from people not accused of any wrongdoing, or to share our most private biological information? Barry Scheck, co-director of the Innocence Project and a member of the New York State Commission on Forensic Science, says regulators have been slow to respond to what he calls rogue databanks. And a recent Supreme Court ruling that defined DNA-gathering as a legitimate police practice comparable to fingerprinting is likely to encourage more freelancing. Scheck says his fear is that misuse will arouse public fears of government overreach and discredit one of the most valuable tools in our justice system. “If you ask the American people, do you support using DNA to catch criminals and exonerate the innocent, everybody says yes,” Scheck told me. “If you ask, do you trust the government to have your DNA, everybody says no.”