It reveals to a golfer who he is and tells him the truth — however unpalatable — about the state of his game.
The Open tells no lies because there’s no smoke to go with the mirrors. A player either posts a score or he doesn’t; there’s no pretending, as Luke Donald discovered when he carded a 7-over-par 77 that featured a birdie on his last hole.
Martin Kaymer, it can be safely assumed, feels pretty good about his game, opening with a Pinehurst No. 2-record 5-under-par round of 65 that he later called "an exceptional score" on Thursday to take a three-shot lead.
"It’s all about the confidence," Kaymer said.
Winning The Players last month filled him with it, and it showed on Thursday.
"It’s always nice if you don’t think much about technique, and just focus on the main things — the yardages, and where you want to pitch the ball, and not thinking too much," he said. "It’s pretty much very straightforward thinking, very positive thinking.
"Today, I hit a couple 6-irons on 15 and 16 that were too good. I hit them so pure that they were, unfortunately, behind the green. I’m actually happy about the shot, even though it ended up in a bad position, but it was a great golf shot. So I see things very positive right now. There’s not much negative. And I really enjoy playing golf that way."
He was, indeed, in the mythical "zone." Of his six birdies, only one came from outside 4 feet.
A natural fader of the ball, his faith in his ability is so high that he hit a towering draw with a 6-iron into the difficult 17th and made a 10-footer for birdie on one of Pinehurst’s toughest holes.
He understands there’s a long way to go. But he’s also prepared for the test and, importantly, he likes his chances.
There were others, too, who brought belief to the first day of the national championship.
Among them, Phil Mickelson.
He has only one top-10 finish on the PGA Tour since he won last year’s British Open but had faith that his short game would keep him in the thick of the race here because he knew the notorious turtle back greens would make life difficult for players.
And he was right.
Mickelson shot an even-par 70 — never a bad score at an Open — and if he can find the answer to his putting woes — he took 31 on Thursday, six more than Kaymer — then he knows he’s really in with a chance at that elusive career grand slam.
"It’s a good start. I didn’t hurt myself any," the soon-to-be 44-year-old said. "I had a chance to get 3, 4, 5 under today had I made some makeable opportunities, but I didn’t throw anything away on some of the short ones."
Mickelson, often erratic off the tee, was exemplary with the driver on Thursday, which gives him confidence heading into the next three days.
"I didn’t miss a fairway with my driver; it’s an unusual thing for me," he said.
"The driver feels really good. I think that’s the club that is going to help me play well this week. And the one club that’s hurting me is the putter. So I’ve got to get that turned around the next couple of days."
Part of his confidence, he said, came from the comfort he feels at Pinehurst.
"I get a similar feeling that I get at Augusta where I don’t have to be perfect," Mickelson said.
"I can miss shots. I can miss greens and still get up and down. I always have a chance. There’s not the hack-it-out-rough. It is challenging. There are difficult shots, but they’re manageable and hittable if you pull them off."
Mickelson won’t allow himself to look too far ahead, but he acknowledged that, after a good start, his dreams of a first US Open victory after a record six runner-up finishes have hardly been dashed.
"This is a special week. This is a special tournament, a tournament that means a lot to me," he said.
"I don’t know if it will be this week or next year or the year after. I do still have a hundred percent confidence that I’ll be able to break through and get one. I do feel, though, that this tournament gives me a great chance."
The mirror, however, has many faces.
There were a host of players who succumbed to the challenge of Pinehurst No. 2 on Thursday.
Bubba Watson, the Masters champion, didn’t like the setup here from the moment he arrived. And, really, who was surprised that he shot an opening 76?