James Madison told the Virginia ratifying convention, "There are more instances of the abridgment of the freedom of the people by gradual and silent encroachments of those in power than by violent and sudden usurpations." Heeding this cautionary note, born of experience, Americans have approached the war on terror with caution and watchfulness.

Those who drafted the Constitution designed the federal government to be a government of "few and defined" powers. Yet the fear of governmental excess was so great that the ratifying conventions in several states sought a bill of rights to make the people's freedom from federal power explicit. Among the freedoms thought most dear was a prohibition on the use by government agents of "general warrants"-that is, authority to search while lacking any articulated specification of the person, place, or thing to be searched or seized. The fear of a generalized police power was no mere hypothetical: when a bag of commercial goods went missing in Wilton, Connecticut, the police obtained a warrant from a magistrate authorizing a search for the stolen bag in every shop, store, and barn in the city. The state supreme court found the warrant so facially overbroad that it upheld a trespass damage award against the issuing magistrate and the police who executed the warrant. Our earliest history thus reflects a firm rejection of the unfettered liberty of government agents to search and seize the citizenry or their possessions when, where, and as they please.

Yet the trend in today's America is an expansion of police power and the curtailment of individual liberty. All can see the change in the conduct of government agents since September 11. No one may enter the Capitol without consenting to an intrusive search of his bags. Congress is considering repealing the Posse Comitatus Act and authorizing the use of military personnel as police officers. Suspected enemy combatants are detained indefinitely and the government argues that its decision to do so is not subject to judicial review. Or, to put it most prosaically, can anyone imagine having to take off his or her shoes to board an airplane in August 2001?

Nobody would seriously dispute the major premise of Judge Posner's recent contribution, "The Truth about Our Liberties" [Summer 2002]: in assessing the appropriateness of infringements on American liberty we must take into account the severity of the threat being averted. In this time of terror, some adjustment of the balance between liberty and security is both necessary and appropriate. And the Constitution is sufficiently malleable and pragmatic to accommodate this balancing of interests. Indeed, the very text of the Fourth Amendment-with its prohibition only of "unreasonable" searches and seizures-implicitly recognizes the need to balance the harm averted against the extent of governmental intrusion.

But in combating the increased threat to public safety we risk systematically undervaluing the countervailing liberty interest. Our history suggests precisely why this risk exists-the insidious contraction of liberty results from measures taken with the best intentions, not malevolent ones. As Judge Posner writes, at the time the internment of Japanese Americans seemed like a reasonable attempt to ensure public safety. Yet, in retrospect, all agree that in placing so great a priority on public-safety interests the government acted unjustly and without sufficient regard for the liberty interests of the Japanese-American citizens.

It may well be that liberty must be curtailed when the public need is great enough. But our history teaches us that we should interpret the Constitution as embodying a cautionary rule: public safety should be effectuated through the least intrusive means possible, allowing maximum scope for personal liberty.

How, then, should we approach the practical questions of governmental conduct arising in a post-September 11 world? With our eyes wide open and with a dose of healthy skepticism. The good news is that we have plenty of both. Courts and the Congress are casting a jaundiced eye at the administration's more extravagant and overblown proposals for reform, while accommodating and expediting the more urgent and reasonable requests. Already, for example, the courts have rejected governmental claims to the right to keep the identities of detainees secret and begun to scrutinize the indefinite detention of individuals as material witnesses or unlawful combatants. The press has accepted the challenge of fulfilling its traditional function as a check on authoritarian excess. Most importantly, the pendulum of public opinion has steadied as the initial shock of terrorism wears off. The American public instinctively understands that prudential adjustments during times of crisis do not (and should not) reset the balance between liberty and security permanently. Once the necessity of war has lapsed, we anticipate a return to the general rule of constitutional liberty.

So long as we keep a vigilant eye on police authority, so long as the federal courts remain open, and so long as the debate about governmental conduct is a vibrant part of the American dialogue, the risk of excessive encroachment on our fundamental liberties is remote. The only real danger lies in silence and leaving policies unexamined.

Thomas Jefferson said, "The natural progress of things is for liberty to yield and government to gain ground." While accommodating the need for government to ensure domestic tranquility in these troubled times, a watchful America can guard against this natural tendency.