(Corrects 17th paragraph with name of Supreme Court case.)

It’s Justice Anthony Kennedy’s country -- the rest of us just live in it. Or so it sometimes feels when the U.S. Supreme Court’s most important and decisions come down from Mount Olympus, aka 1 First Street, NE, where the justices preside in their white marble temple in Washington.

The most recent 5-4 decision with Kennedy writing the majority opinion ordered the release of some 46,000 inmates from California prisons over the next few years unless the state can find room for them in alternative county lockups.

Justice Antonin Scalia fulminated shockingly on the danger of releasing “fine physical specimens who have developed intimidating muscles pumping iron in the prison gym” onto the unsuspecting populace. But dissent was all Scalia could muster: With the court’s four liberals behind him, Kennedy couldn’t be stopped.

Sometimes, of course, Kennedy is on the conservative side. Last month, it was Justice Elena Kagan who was outraged when a 5-4 Kennedy opinion, joined by the four conservatives, made it much harder for taxpayers to sue when the government violates the separation of church and state.

It is Kennedy’s apparent unpredictability -- and his willingness to make common cause with both factions in different cases -- that is the source of his overwhelming power in court and country. This year, there have been nine 5-4 cases; Kennedy has been in the majority every time. (Last year he was the controlling vote in 12 of 17 cases decided 5-4; the previous year 20 out of 25.)

Not Another O’Connor

For years, Justice Sandra Day O’Connor played a similar role as a swing voter. A former state legislator, O’Connor was a pragmatic centrist, sniffing the wind to get a sense of where the median voter wanted to go, and taking the court there with her. But even at the height of her centrist power she never quite reached the position of utter dominance that Kennedy now enjoys -- partly because Kennedy was on the court with her, so she was not the only wild card.

Justice Kennedy is different. His opinions tend to be grounded on strong statements of principle. Yet many find his tacking from right to left mystifying, frustrating and unpredictable. They question what consistent principles could guide such apparently disparate conclusions, and hint darkly at incoherence or self-aggrandizement.

The truth is that Kennedy’s principles can be identified and explained; and that explanation can help predict what Kennedy will do next in all-important cases, such as the constitutionality of the health-care law or gay marriage.

Well-Defined Principles

As it happens, some of Kennedy’s principles were on display in the California prisons case. The facts were simple: California’s prisons are at roughly 200 percent of capacity. Physically and mentally ill inmates sued for adequate services. Two courts over the last decade ordered improvements. When nothing happened, a special three-judge federal district court ordered that the facilities be reduced to 137.5 percent of capacity.

In reviewing the lower court’s finding, Kennedy started with what is undoubtedly his favorite constitutional concept: dignity. Prisoners, he said, have lost their liberty, yet they maintain their dignity as human beings. In fact, Kennedy asserted, the whole point of the Eighth Amendment’s ban on cruel and unusual punishment is to preserve human dignity.

Once dignity was in play, Kennedy was essentially guaranteed to hold in favor of the inmates. He has invoked the principle in a bewildering array of contexts, always with decisive force. Dignity was at the heart of Kennedy’s vote to preserve the basic abortion right in the famous Planned Parenthood v. Casey decision, where he wrote of the need to respect “choices central to personal dignity and autonomy.”

Unlimited Dignity

Dignity appeared again when Kennedy, in the landmark case of Lawrence v. Texas, found a right to choose one’s sexual partner and so struck down an anti-sodomy statute.

More recently, Kennedy invoked dignity to uphold restrictions on so-called partial-birth abortions, reasoning that the congressional ban expressed “respect for the dignity of human life.” Perhaps most strangely, Kennedy relied on the dignity of state government to conclude that states couldn’t be sued for money damages in their own courts, even if they broke federal law.

Dignity, of course, can mean many things. The dignity to choose is a kind of freedom, but prisoners have lost that sort of dignity. Their kind of dignity is the basic right to be treated humanely. The dignity of a fetus was, in Kennedy’s view, connected to the method of abortion. And whatever dignity is possessed by state governments must be something very abstract indeed. The key point, though, is that Kennedy takes dignity seriously.

The Best Argument

Anyone who wants to win his vote would do well to argue that someone’s dignity is being violated somewhere. The California inmates had a pretty good argument on that front, both in terms of the Constitution and the facts of their treatment.

There was another of Kennedy’s pet principles at work in the prison decision: the vindication of the power of the judiciary. For straightforward political reasons -- who is the constituency interested in improving prisons? -- California had for years been flouting court orders demanding reform in prison health care.

If there is one thing that enrages Justice Kennedy, it is seeing the courts ignored by other branches of government. In 1999, after Congress passed legislation reversing a Supreme Court opinion on religious liberty, Kennedy wrote the opinion in City of Boerne v. Flores rejecting Congress’s efforts as illegitimate.

Castigating Congress

In 2008’s Boumediene v. Bush, the most dramatic of the Guantanamo Bay cases, Kennedy -- as usual writing for a 5-4 majority -- castigated Congress and the president for excluding the courts from supervising detention in the war on terror. “The Nation’s basic charter,” Kennedy wrote, “cannot be contracted away like this.” In other words, Kennedy was saying, the court couldn’t trust the other branches to apply the Constitution without it.

Given his commitment to dignity and judicial authority, Justice Kennedy’s vote in the California prisons case wasn’t unpredictable at all. Similarly, one can predict that when it comes to gay marriage, Kennedy will believe human dignity means everyone should have equal marriage rights regardless of his or her sexual orientation.

As for health care, the question is closer: Does mandatory insurance violate individual dignity-as-choice? Or does human dignity require universal health care? Justice Kennedy’s answer will be the law of the land.

(Noah Feldman, a law professor at Harvard, is a Bloomberg View columnist. The opinions expressed are his own.)

To contact the writer of this column:

noah_feldman@harvard.edu

To contact the editor responsible for this story: