When the Supreme Court ruled last month that the Fourteenth Amendment requires states to issue marriage licenses to same-sex couples, we were reminded that the American people remain divided not only about gay marriage but also about the very nature of our Constitution.

The conservatives view the Constitution as a law that should be interpreted and applied the way other laws are.

The ordinary provisions of the civil and criminal law derive their authority from the fact that they were enacted by the democratic process, and judges are bound to apply them faithfully, even when they think them unwise or unjust. Many judges have spoken out against mandatory minimum sentencing laws, but as long as they are on the books, judges can’t ignore them.

Nor, for the same reason, do they get to create new laws required by changing times. There was a notorious incident in Massachusetts a couple of years ago when a man held his cellphone under a woman’s skirt to take pictures of her underwear. His conduct was outrageous, but not a violation of any law then on the books, so the courts ultimately found the man innocent of any crime. Naturally, the state legislature promply outlawed the practice.

To the conservatives on the Supreme Court, the Constitution works the same way. It is binding because it was ratified democratically. Judges, therefore, have to follow what the Constitution says, even when they think it is unwise or unjust. And they sometimes find themselves compelled to acknowledge that the Constitution doesn’t speak at all to issues that we care a lot about today.

In their view, the gay marriage case was an easy case, which the majority got obviously wrong. The Fourteenth Amendment means today what it meant in 1868 when it was ratified. No one at the time would possibly have imagined that when the people ratified that amendment they were approving same-sex marriage, so there isn’t any right to gay marriage in the Constitution. And, as Chief Justice Roberts stressed, this conclusion has nothing to do with whether or not one thinks that same-sex marriage is a good or bad thing.

To Justice Kennedy, by contrast, the Constitution is more than an ordinary law, and constitutional adjudication is fundamentally different from ordinary statutory interpretation. In his view, the Constitution’s authority derives, not so much from its having been voted on long ago by people who are now all dead, but from the fact that people in the present accept it as legitimate. Moreover, he holds that people can only accept it as legitimate today insofar as it embodies moral prinicples that people, today, should and do in fact embrace.

Hence, in his view, the meaning of the Constitution does and should evolve over time. Rights, he states in his opinion for the Court, do not come from “ancient sources alone.” Instead, they come from “a better informed understanding” of liberty. Thus he devotes the bulk of his opinion to arguing that it is morally imperative that all the states confer the right to marry upon couples of the same sex.

Kennedy is right that the Constitution’s authority derives from the fact that we, who are living today, accept it as legitimate. One difficulty in his view is that conservatives and progressives accept the legitimacy of the Constitution for very different reasons — the conservatives, because it was ratified long ago and not replaced, and the progressives, because they think it is just, or that it can become so as it evolves with changing times.

The other difficulty in Kennedy’s view is that the Constitution does not simply evolve on its own: its text can only be changed by the difficult amendment procedure set out in Article V. For constitutional law to evolve, the Justices have to change it. The result is paradoxical: progressive Justices set themselves against democracy (overturning laws enacted democratically) in order to preserve the democratic legitimacy of the Constitution by causing it to evolve in a direction they expect people to support.

Sometimes, such judge-led evolution succeeds in co-opting conservative opponents, as has happened with the right to contraception. Sometimes, however, it fails: more than 40 years after the Court evolved abortion rights into the Constitution, abortion remains deeply controversial and keeps alive the bitter disagreement between conservatives and progressives about the nature and meaning of the Constitution.

Gay marriage now seems poised to win the overwhelming public support the progressive Justices expect, but that is what they once thought about abortion.

Joseph R. Reisert is associate professor of American constitutional law and chairman of the department of government at Colby College in Waterville.