Yankees closer Mariano Rivera enters Monday’s game in the ninth inning against the Minnesota Twins. He earned his 602nd career save, passing Trevor Hoffman for first place on the career list. (Kathy Kmonicek/AP)

Long before anyone ever heard of Mariano Rivera, baseball had a theory that was passed from generation to generation of pitchers. The best qualities a pitcher could possess were elite speed, quick late movement, ridiculous throw-it-in-a-teacup command and an indestructible arm that never broke.

This was a hypothetical, a mere teaching point. No such pitch had ever existed. No such pitcher had ever been born. And no arm could throw such a dragon’s tongue of a pitch for 20 years and not crack asunder.

Even Walter Johnson, whose 100-mph sidearm fastball perhaps came closest, gradually developed his “nickel curveball,” a precursor to the slider.

But if such a pitch, pitcher and arm, plus a warrior’s spirit could be combined, what would you have? Then, if you added an imperial, entitled presence and a mound demeanor that merited a Metallica anthem as an introduction, then, once again — purely in theory — that pitcher could dominate almost every hitter, win nearly every game of consequence and stand above baseball itself with just that one almost unhittable pitch.

Then unannounced and unexpected in ’95, the Sandman actually entered.

At first, few noticed. Something about Rivera seemed a trifle too humble and soft-spoken to become synonymous with a 15-year reign of domination. He was a neat, polite Panamanian gentleman without a hint of extroversion. But then similar things were said of Johnson when he arrived from the Idaho State League.

No one could possibly throw one pitch, over and over, under the greatest pressure, year after year, playoff and World Series after playoff and World Series and almost never falter. Up and in (in your kitchen), on the fists (broken bat), backdoor low and away (take a seat) and apparently right down the middle, then suddenly on the corner low-and-away to right handed hitters, down and in to the lefties — equally and inevitably fatal to both.

But Rivera could.

No pitcher with 1,000 innings since the ancient Dead Ball Era had an ERA to compare with his career mark of 2.22. The closest, Hoyt Wilhelm (2.52), threw the ultimate trick pitch, the knuckleball.

However, in the postseason Rivera redefined baseball perfection. In 94 games, he won eight times, saved 76 games and lost — once. That defeat, to end the ’01 World Series, was based on Rivera’s own throwing error, not his pitching. The singularity of the event — Mo losing Game 7 — was the final thematic note that, perhaps, places the ’01 Series as the best ever.

As homage to Rivera, and a nod to the temper of this baseball age, let his slash line (batting average/on-base percentage/slugging average) in the postseason take our breath away: .172/.213/.229. On the biggest stage, against the best hitters, he made them all hit like pitchers.

Mariano was the greatest Yankee since Babe Ruth. He was more valuable, more central and more emblematic than Mickey Mantle, Reggie Jackson, Derek Jeter or any of the assorted Hessians like Roger Clemens of the free agent era since ’76. That’s an opinion from observation; it’s not a proof. Joe DiMaggio hit .271 (.760 OPS) in his 10 World Series. Rivera’s ERA was 0.71 in 31 postseason series. Not 31 games, 31 entire series.

On Monday, Rivera broke the last record available to him — 602 career saves. Not that he needed it or that anyone cared about the number itself. But the moment was needed, the finality, the period at the end of the sentence of greatness at the pinnacle of one occupation.

Over the years, Metallica borrowed Rivera just as he had annexed their ’91 hit as his introduction. A huge video of Rivera jogging from the pen to the stadium mound, then baring his teeth on every 97-mph cut fastball, now plays behind Metallica when they crank up “Enter Sandman,” their ode to nightmares that beset the innocent: “Exit light. Enter night. Take my hand. We’re off to Never Never Land.” But one Peter Pan never visited.

Metallica grew old. Rivera, 41, remains as young as his 1.98 ERA, his 11th season under 2.00. He’s the nightmare from which hitters cannot awake.

There are times and places you never forget. You wonder if you can ever re-create them for those who weren’t. Rivera’s entrance at the Big Ballpark — more encores are scheduled for next month — was a human bolt of collective lightning as 50,000 people erupted. Rivera did not symbolize victory. He was the emblem of defeat. The hangman, the expressionless executioner and the ultimate closer in a town that worships closing the deal. He embodied Yankee power, even though they never bought him at auction, just signed and taught him, almost like a normal mortal player with flaws.

Mo was The End. After him, roll the credits.

Until Rivera, perhaps no one ever really believed that old theory — that with speed, late movement and command in all quadrants of the strike zone, the game of baseball was simply too difficult for hitters, even the best. Of late, he’s added wrinkles. But he will be remembered in his long prime as a symbol of dignity, consistency, perfected craft and that aura, which he never possessed until he took the mound, transformed, into controlled menace.

Too soon, Mo will become an oldie-but-goodie. But not yet. The Yanks have the best record in the AL and have outscored their foes by even more than the much-heralded Phillies. Will we have a Series with Roy Halladay, Cliff Lee and Cole Hamels pitching on one side against a rotation of CC Sabathia, Question Mark and the Mysterians, plus ancient Mariano Rivera?

Some say: Just enjoy 602. Don’t expect another big Mo-ment in October. Fine, you believe that. The greatest of the great in all sports will dispute it.

If the Yanks reach the point where they can shut the door on another season, Rivera will arrive, as he always has, like the dream of every pinstripe fan, but the final nightmare for those who must face the Yankees.

Exit light. Enter night. Yankees win. Yankees win.