Sports fans and historians require reference points to define potential, which is why up-and-coming athletes are so often compared to past greats. And if today’s hot prospect somehow lives up to his advance billing, you can bet that at some future date his career, provided it has aged well, will be cited as the standard against which the next up-and-comer will be assessed.

When Roy Jones Jr. was a youthful freak of nature, the fighters to whom he most often was measured were Sugar Ray Robinson and the pre-suspension Cassius Clay/Muhammad Ali. Who else had that mystifying blend of hand speed, nimbleness and reflexes that seemed so otherworldly? To glimpse the early Jones at work was to imagine all of boxing’s possibilities, a rare melding of power and grace that comes along, if we are fortunate, maybe once a generation.

It has been 20 years since I first observed Jones operating on the preferred side of a talent chasm as wide as the Grand Canyon. Befuddled opponents soon discovered that fighting the brash kid from Pensacola, Fla., was like trying to put moonbeams in a bottle, only more painful. As was the case with the lithe and sleek Clay, Jones did everything wrong, like dropping his hands at his sides and pulling straight back from punches, but it turned out right because he had those mad skills. Who needs fundamentals and technique when you’re capable of making things up as you go along? Jones reminded me of jazz genius Miles Davis, riffing to a rhythm in his head only he could hear, producing sounds that trumpet players reading from sheet music couldn’t even imagine.

Now that Jones (52-4, 38 KOs) is 39 and about to throw down with Wales’ Joe Calzaghe (45-0, 32 KOs) in a 12-round non-title light heavyweight bout Saturday night in Madison Square Garden, I still have to admit that RJ is – or, at least, was — the most physically gifted fighter of the past quarter-century. But I no longer use Miles Davis as the benchmark against whom Jones’ eroding magic is best gauged.

Like Mike Tyson, another of his contemporaries who tantalized the public with his might-have-beens, Jones most reminds me of the late, great New York Yankees slugger, Mickey Mantle, because I can only guess at how much of those vast stores of ability was actually utilized by any of them.

See, no one disputes that The Mick is a Hall of Famer on merit. Those 536 home runs, a record 18 of which came in World Series play, tell us as much. The unfathomable power that produced a 565-foot home run off Washington Senators lefthander Chuck Stobbs and a glut of other jaw-dropping tape-measure blasts also is proof of Mantle’s prowess. When he still had his legs, the switch-hitting Mantle was timed in 3.1 seconds from batter’s box to first base on a drag bunt from the left side. That’s not just fast, it’s almost Usain Bolt-fast.

But, as we learned in Jim Bouton’s tell-all book, “Ball Four,” Mantle sometimes was a hero with clay feet. He so liked the nightlife that he hit the bottle nearly as hard as middle-in fastballs, and there were occasions when he reported to the ballpark more than a little hung over. Because of a degenerative bone condition, osteomyelitis, and the effects of a knee ripped up when his spikes got caught in a Yankee Stadium drainage cover, that Adonis of a body gradually betrayed him until he needed yards of tape and bandages to protect all the areas that hurt. Aware of a family predisposition to cancer, a disease which had claimed the lives of several coal-mining Mantle men at an early age, No. 7 lived fast and hard, as if he were forever racing against the clock.

“If I’d known I was gonna live this long, I’d have taken a lot better care of myself,” Mantle, who died in 1995 at the age of 63, often joked.

Mantle fans, aware of the infirmities through which their idol battled for much of his career, have romanticized what he might have accomplished with a healthy set of wheels and a stricter adherence to clean living. Would he have hit 700 home runs? Eight-hundred? Might he have so separated himself from other baseball legends that he would now stand alone at the top of the sport’s very highest mountain?

Jones is no Mantle in several discernible ways. He is not afflicted with any hard-to-spell disease, for one thing. I’m not sure if he likes to take the occasional drink, but I’ve heard no such rumors to that effect. And when he peels off his shirt, those washboard abs still look taut enough to do your grandmother’s laundry.

But, as was the case with Mantle and with Tyson, another fighter who gave us so much but probably not nearly all that he was capable of delivering, Jones has gone only part of the way his talent should have carried him. He has been boxing’s perpetual tease, hinting at a Himalayan level of greatness that his actual accomplishments in the ring haven’t always risen to.

I’m not a psychologist, so I won’t attempt to go all Freudian in analyzing why Jones has been as disappointing in some regards as he has been exhilarating in others. I do think he has harbored a fear of being seriously hurt because of the state of living death in which his friend, Gerald McClellan, has existed for these past 13 years. It’s a gut reaction that most human beings can understand; prizefighting is a dangerous occupation most sensible persons wouldn’t dare attempt.

It does seem apparent to me, though, that Jones is a mass of conflicted emotions, a preening show of bravado on the outside and a gnawing core of self-doubt on the inside. Teddy Atlas told us years ago, before Mike Tyson’s comeuppances at the hands of Buster Douglas, Evander Holyfield, Lennox Lewis, Danny Williams and even Kevin McBride, that the self-proclaimed baddest man on the planet was a bully who would not know how to react when someone had enough gumption to stand up to him.

I have seen the best of Jones, and it was very good indeed. I was there at ringside in Atlantic City when he embarrassed Vinny Pazienza by landing eight left hooks in a row, a burst of brilliance that seemingly took no more than two eye-blinks from start to finish. I was there at ringside in Biloxi, Miss., when he put Virgil Hill down and out with a body shot that veteran HBO analyst Larry Merchant said “sounded like a whip cracking against a saddle.”

I have also seen Jones’ other signature victories, although, in retrospect, they come with caveats. He outboxed Bernard Hopkins in 1993, which was before a still-evolving Hopkins had fully mastered his craft, and James Toney in 1994, which was the result in no small part because Toney somehow gorged his way into a 24-pound gain increase in the short time from the weigh-in to the opening bell. Maybe Jones would have made Toney look slow anyway, but all that late heft surely contributed to “Lights Out’s” sluggishness.

Oh, and that 2003 decision over WBA heavyweight champion John Ruiz, which some feel is the cornerstone of the Jones legend? Despite the significant size disparity (Ruiz weighed 226 pounds to Jones’ 193), it now seems apparent that Jones selected Ruiz because the big man’s propensity for clutch-and-grab tactics represented the least dangerous heavyweight option for a man who had increasingly come to known as “Reluctant Roy.”

But even on those occasions when Jones falls upon hard times – most notably the three-fight losing streak, with defeats (one by knockout) to Antonio Tarver sandwiched around a brutal beating and stoppage at the hands of Glen Johnson – Jones has publicly maintained the veneer of the invincible destroyer, a masterpiece of pugilism who is as awe-inspiring as ever.

Asked if a victory over Calzaghe would lead to the long-delayed rematch with Hopkins, Jones reacted as if such a possibility was beneath his consideration.

“I already beat Bernard Hopkins, so why would I want to fight him again?” Jones said with the familiar imperiousness.

Asked why his resume does not include some names that should be on it, Jones said, “I had my reasons for doing what I was doing. I made (boxing) so delightful that you didn’t need to watch me fight another `name’ fighter. I could fight anybody. You knew when Roy Jones went into the ring he was going to do something spectacular. When (Kelly) Pavlik stepped into the ring with Bernard Hopkins, the people knew they were going to get a regular old fight. Now, when Roy comes, it’s going to be a show. Not only is he good and is going to beat the hell out of who he is fighting, but he is going to look good doing it.”

Such bravado from a fighter who has fought only seven times in the last five years, is 4-3 in those bouts and hasn’t won inside the distance since he took out Clinton Woods in six rounds on Sept. 7, 2002. Two of his victories since then have come on points against Prince Badi Ajamu in Boise, Idaho, and Anthony Hanshaw in Biloxi, Miss., which might kindly be described as minor successes against B-list opponents in C-list venues.

Watching Jones now is like watching Mickey Mantle limp up to the plate in the late-1960s. You fondly remember what was, forlornly shake your head at what might have been, and resign yourself to what is. Put it this way: From a technical standpoint, Jones still does everything wrong, but now it doesn’t always turn out right.

In April 2006, when Jones was mulling whether he would fight again, I asked former HBO Sports president Seth Abraham for his thoughts on this enigma wrapped in a riddle.

“His drive was to do things that were of interest to him, but not necessarily to fight the very best middleweights, super middleweights and light heavyweights who were out there,” Abraham responded. “I think Roy’s legacy in the sport absolutely will suffer because he chose not to do everything he could to make himself as great as he might have been.”

At least Mantle had osteomyelitis as an excuse for never being quite all that he could have been.