WASHINGTON -- The day after the Supreme Court ruled that detainees imprisoned at Guantanamo are entitled to seek habeas corpus hearings, John McCain called it "one of the worst decisions in the history of this country." Well.

Does it rank with Dred Scott v. Sanford (1857), which concocted a constitutional right, unmentioned in the document, to own slaves and held that black people have no rights that white people are bound to respect? With Plessy v. Ferguson (1896), which affirmed the constitutionality of legally enforced racial segregation? With Korematsu v. United States (1944), which affirmed the wartime right to sweep American citizens of Japanese ancestry into concentration camps?

Did McCain's extravagant condemnation of the court's habeas ruling result from his reading the 126 pages of opinions and dissents? More likely, some clever ignoramus convinced him that this decision could make the Supreme Court -- meaning, which candidate would select the best judicial nominees -- a campaign issue.

The decision, however, was 5-4. The nine justices are of varying quality, but there are not five fools or knaves. The question of the detainees' -- and the government's -- rights is a matter about which intelligent people of good will can differ.

The purpose of a writ of habeas corpus is to cause a government to release a prisoner or show through due process why the prisoner should be held. Of Guantanamo's approximately 270 detainees, many certainly are dangerous "enemy combatants." Some probably are not. None will be released by the court's decision, which does not even guarantee a right to a hearing. Rather, it guarantees only a right to request a hearing. Courts retain considerable discretion regarding such requests.

As such, the Supreme Court's ruling only begins marking a boundary against government's otherwise boundless power to detain people indefinitely, treating Guantanamo as (in Barack Obama's characterization) "a legal black hole." And public habeas hearings might benefit the Bush administration by reminding Americans how bad its worst enemies are.

Critics, including Chief Justice John Roberts in dissent, are correct that the court's decision clouds more things than it clarifies. Is the "complete and total" U.S. control of Guantanamo a solid-enough criterion to prevent the habeas right from being extended to other U.S. facilities around the world where enemy combatants are or might be held? Are habeas rights the only constitutional protections that prevail at Guantanamo? If there are others, how many? All of them? If so, can there be trials by military commissions, which permit hearsay evidence and evidence produced by coercion?

Roberts' impatience is understandable: "The majority merely replaces a review system designed by the people's representatives with a set of shapeless procedures to be defined by federal courts at some future date." Ideally, however, the defining will be by Congress, which will be graded by courts.

McCain, co-author of the McCain-Feingold law that abridges the right of free political speech, has referred disparagingly to, as he puts it, "quote 'First Amendment rights.'" Now he dismissively speaks of "so-called, quote 'habeas corpus suits.'" He who wants to reassure constitutionalist conservatives that he understands the importance of limited government should be reminded why the habeas right has long been known as "the great writ of liberty."

No state power is more fearsome than the power to imprison. Hence the habeas right has been at the heart of the centuries-long struggle to constrain governments, a struggle in which the greatest event was the writing of America's Constitution, which limits Congress' power to revoke habeas corpus to periods of rebellion or invasion. Is it, as McCain suggests, indefensible to conclude that Congress exceeded its authority when, with the Military Commissions Act (2006), it withdrew any federal court jurisdiction over the detainees' habeas claims?

As the conservative and libertarian Cato Institute argued in its amicus brief in support of the petitioning detainees, habeas, in the context of U.S. constitutional law, "is a separation of powers principle" involving the judicial and executive branches. The latter cannot be the only judge of its own judgment.

In Marbury v. Madison (1803), which launched and validated judicial supervision of America's democratic government, Chief Justice John Marshall asked: "To what purpose are powers limited, and to what purpose is that limitation committed to writing, if these limits may, at any time, be passed by those intended to be restrained?" Those are pertinent questions for McCain, who aspires to take the presidential oath to defend the Constitution.