“Umpires are eyewitnesses,” said Jim Evans, a major league umpire from 1972 to 1999 who now runs a school for umpires in Florida. “As the umpire you are the eyewitness and the judge. You make your decision based on your own reportage.”

Image THE DECISION Jackie Robinson is safe at home, rules the umpire, Scotty Robb. Credit... Ray Howard/Associated Press

Sure, umpires call ’em as they see ’em, and judges learn about everything from interested parties; they call ’em as they hear ’em. In a 2007 case, however, Scott v. Harris, the justices played umpire and ruled on the basis of what they saw. The case involved a motorist, Victor Harris, who was fleeing the police and was rendered quadriplegic after a police car rammed him to put an end to a high-speed chase. After viewing a videotape of the chase, the court ruled 8-1 that Harris was not entitled to sue on the basis of his Fourth Amendment protection against unreasonable search and seizure. “No reasonable juror,” Justice Antonin Scalia wrote for the majority, could fail to see the deadly risk to the public posed by Mr. Harris’s flight. And rather than publish a rebuttal of the minority opinion of Justice John Paul Stevens, the court chose instead a unique response; it posted the videotape on its Web site.

“We are happy to allow the videotape to speak for itself,” Justice Scalia wrote.

A subsequent study by Dan M. Kahan, David A. Hoffman and Donald Braman of what 1,350 people saw in the video yielded startling results. “Whites and African- Americans, high-wage earners and low-wage earners, Northeasterners and Southerners and Westerners, liberals and conservatives, Republicans and Democrats  varied significantly in their perceptions of the risk that Harris posed,” they wrote in the Harvard Law Review in January.

So what is the umpire’s objective judgment here? Where is the foul line?

Another difference between umpires and Supreme Court justices is that umpires can be reversed. Perhaps the most famous example of this is the pine tar incident of 1983. George Brett of the Kansas City Royals hit an apparently decisive ninth-inning home run against the Yankees but was declared out by the umpires for having too much pine tar on his bat. The umpires were in strict constructionist mode; the rulebook said that the use of an illegal bat would cause the batter to be declared out and Mr. Brett had pine tar on his bat well beyond the limit of 18 inches above the handle. But the Royals protested, and the American League president upheld the protest  citing the spirit rather than the letter of the law  and the game resumed. The rule, which had been implemented in the first place to discourage a different illegality  bat corking  was rewritten to say that a bat with too much pine tar would result not in an out but in the bat’s being removed from play.

Umpires tend to be politically conservative  “Maybe we should have Latina umpires because they have more empathy, right?” Mr. Evans suggested  but the pine tar example notwithstanding, they often view their role on the field more progressively. Of course, it might just be they like exercising their authority, but in many matters  calling balls and strikes, for instance  umpires are loath to yield their personal discretion.

“It’s like the Constitution,” the umpire Gary Cederstrom said to me. “The strike zone is a living, breathing document.”

Last week, the umpire Marty Foster called the Yankees’ Derek Jeter out on a steal of third, and though it appeared he was never tagged, Mr. Jeter said Mr. Foster explained that he didn’t need to be tagged to be called out because the ball beat him to the bag. Talk about judicial activism! An uproar arose over this, but in fact, if that’s what Mr. Foster said, he was simply  if unwisely  expressing aloud a generally unspoken umpire tenet that allows for some discretion on close plays to keep managers and fans, who can clearly see throws but not tags from the dugout or the stands, from causing a ruckus.