... The idea that a judge should consider evidence about a law or a ruling’s effects has been a central tenet of legal thinking since at least the early 20th century. In that period, scholars known as legal realists challenged the opposing conception — usually called formalism — that law should logically derive from abstract principles. The realists wanted judging to be grounded pragmatically in evidence about consequences. Holmes was a protorealist: “The life of the law has not been logic: It has been experience,” he said, framing the law as “a reaction between tradition on the one side and the changing desires and needs of a community on the other.”

Today the debate between legal realism and formalism is entangled in politics. Formalists say that realism (or pragmatism, a related concept) is just a cover for a judge to impose his or her own ideological values. They argue that it’s the job of the legislature, not the judiciary, to take into account real-world consequences. Justice Antonin Scalia, the country’s foremost formalist, believes that the only way for judges to guard against ideological temptation is to apply his fixed theory of interpreting the text of a law based on its original meaning, especially when it comes to the Constitution. Scalia says that his approach allows him to separate his legal decision-making from his political inclinations. His critics say that formalism is equally political and that Scalia and other originalists have chosen a theory that helps them achieve their conservative goals.

Scalia’s main sparring partner in this fight is Judge Richard Posner, whom President Ronald Reagan nominated to the United States Court of Appeals for the Seventh Circuit. Posner is not a liberal, so he and Scalia don’t divide along the usual ideological lines. Instead, their argument is about whether it is really possible to separate law from politics. Posner, a self-defined pragmatist, is probably Holmes’s closest intellectual heir today. As a professor at the University of Chicago, where he still teaches, he was an early contributor to the law-and-economics movement, which tests legal rules against their predicted economic consequences. In the 1970s, Posner’s scholarship had a part in persuading the Supreme Court to become more hospitable to big business in the domain of antitrust law by showing that some market structures that appear to reduce competition could actually benefit consumers.

For years, Posner has argued that judges should be less concerned about theories or past precedents and “more open to the facts,” as he told me. “Most judges start with the old stuff,” he said. “But they should ask themselves, given modern conditions, what is the right result? When I get a new case, I ask myself, ‘What is the common-sense solution here?’ ”

If past precedents pose an obstacle, Posner argues, a judge can usually get around them by distinguishing the facts in the previous case from the current ones. He acknowledges that ideology, in the sense of moral and political values, will affect what a judge sees as “the right result.” To him, though, this is not a knock on pragmatism, but simply an “inescapable feature of legal judgment,” especially in the divisive cases that reach the Supreme Court. His solution is a diverse judiciary that reflects the political spectrum of the country.

These two ways of thinking, Scalia’s and Posner’s, are particularly relevant now as we await the Supreme Court’s decision on same-sex marriage....