Supreme Court Justice Anthony Kennedy’s retirement became official on July 31, 2018. That means there’s a position that needs to be filled. As in any other profession or job category, hiring someone requires a job description. Without that, it’s pretty much impossible to know who’s right for the job or, if they get hired, to evaluate their job performance.

What is the job description for a federal judge or, more specifically, for a Supreme Court justice? During his September 2005 confirmation hearing before the Senate Judiciary Committee, Chief Justice John Roberts offered this description:

Judges are like umpires. Umpires don’t make the rules; they apply them. The role of an umpire and a judge is critical. They make sure everybody plays by the rules. But it is a limited role. Nobody ever went to a ball game to see the umpire.

The umpire analogy actually has a longer historical pedigree. In 1934, Sherman Minton was elected to the U.S. Senate as a Democrat from Indiana. He served one term before President Franklin Roosevelt appointed him to the U.S. Court of Appeals. Then, in September 1949, President Harry Truman nominated him to the Supreme Court.

The Judiciary Committee wanted Minton to appear at a hearing to discuss his views on the 1937 legislation that would have expanded the number of Supreme Court justices. Minton declined to appear. In a letter dated October 1, 1949, to the Judiciary Committee chairman, he wrote: “When I was a young man playing baseball and football, I strongly supported my team. I was then a partisan. But later, when I refereed games, I had no team. I had no side. The same is true when I left the political arena and assumed the bench.”

The Left sees judges as players rather than umpires. These political judges are what Minton rejected, partisans who favor one team (a.k.a., party) over another. This is what Senator Ted Kennedy (D., Mass.) had in mind when he opposed the Roberts nomination in 2005 and said that “we need to know whose side Mr. Roberts is on.”

Judge Brett Kavanaugh has embraced the parallel between a judge and an umpire. In March 2015 speech at Catholic University School of Law, for example, he outlined ten principles by which a “good judge” is like an umpire. “To be an umpire as a judge,” he said, “means to follow the law and not to make or re-make the law — and to be impartial in how we go about doing that.”

No one confuses an umpire who impartially applies the rules with the players who are trying to win the game. The difference is obvious. And it takes just a bit of common sense to see which role — umpire or player — is the best model for judges. For at least seven decades this analogy has proved useful to understanding the judicial job description. Senators should use it now as they decide who should fill this Supreme Court vacancy.