In a recent interview with the Washington Post, retired Supreme Court Justice John Paul Stevens, whose memoir is being released tomorrow, shows why the conflict over the kind of judge a president appoints is so important.

President Richard Nixon appointed Stevens, who turned 99 last month, to the U.S. Court of Appeals in 1970 and, five years later, President Gerald Ford appointed him to the Supreme Court. The Senate confirmed him to both judicial positions without any opposition. Stevens is the third longest serving member of the Supreme Court in American history.

Judges decide cases by interpreting and applying law to the facts and can best be evaluated by the process they follow in those tasks. Impartial judges interpret the law, such as the Constitution or statutes, as written and apply it without regard for which side wins or which interests might be advanced. Political judges interpret the law as they want it to be and apply it with an eye toward which party might benefit or the impact of their decisions.

Stevens’s Post interview made clear which side he’s on. He said that the 2008 decision in District of Columbia v. Heller was “[u]nquestionably the most clearly incorrect decision that the Supreme Court during my tenure on the bench.” In Heller, the Supreme Court voted 5-4 to embrace the “original understanding of the Second Amendment.” That amendment, the court said, protects an individual’s right to possess a firearm and to use it for traditionally lawful purposes such as self-defense.

The problem is not that Stevens thinks Heller was wrongly decided; after all, he wrote the primary dissenting opinion. No, the most important thing, as it is for every judge in every case, is the basis for that conclusion. Stevens revealed it in his Post interview.

“I think,” he said, “there’s no need for all the guns we have in the country and if I could get rid of one thing, it would be to get rid of that whole gun climate . . . [I]t seems to me we don’t have to have this kind of thing in this country, and we should do everything we can to try to change it.”

There you have it. The Heller decision was “clearly incorrect,” according to Stevens, because it has “such disastrous practical effects.” In fact, he was so driven by the effects he anticipated that he circulated his dissenting opinion weeks before the majority opinion was finished. In other words, he lobbied his colleagues to “do everything we can” to “get rid of that whole gun climate.”

But when the Constitution divides government power into three branches, to which does it give the power to decide what kind of climate we want in this country? Which branch has the power to consider the effects of different policies? Stevens, and judges like him, believe that the answer includes the one branch that the people do not elect.

The idea that any five members of the Supreme Court can, as Thomas Jefferson wrote, treat the Constitution like a “mere thing of wax,” twisting it into the form they need to accomplish their ends, is incompatible with the liberty our system of government was designed to provide. This is why the conflict over the appointment of judges is a conflict over the power of judges.

Daniel Webster, Senator from two states and Secretary of State under three presidents, said in 1837 that “good intentions will always be pleaded for every assumption of authority. It is hardly too strong to say that the Constitution was made to guard the people against the dangers of good intentions. There are men in all ages who mean to govern well, but they mean to govern. They promise to be good masters, but they mean to be masters.” Judges must never be our masters.