To be considered for entrance into the Boxing Hall of Fame, it is almost a necessity that a fighter be blessed, and cursed, with a rival who gave as good as he got, who pushed him to unforeseen heights and pummeled him to savage depths.
Muhammad Ali needed Joe Frazier to shove him to another sphere of savage majesty, and the two entertained customers as they jostled for supremacy three times.
Other fighters, like Floyd Mayweather, are still searching for the foes who own a like reservoir of weaponry, physical and mental, who can push their engines to full throttle.
There aren't many boxers who are fortunate enough to find themselves in more than one signature rivalry, but Mexican fight icon Erik Morales (48-4, 34 KOs) can boast two.
He has traded blood and demonstrated Canastota-level guts in his three fights with countryman Marco Antonio Barrera. On Saturday, Morales will step in with his other super-rival, the Filipino whirlwind Manny Pacquiao.
Morales, who's only 30 but who seems older because he's waged 52 wars since turning pro in 1993, has faced off twice with the human tsunami, Pacquiao (42-3-2).
Morales, the Tijuana-born hitter who answers to El Terrible, was coming off a loss to his principal rival, Barrera, when he met Pacquiao in Las Vegas on March 19, 2005. Morales had his hand raised, by scores of 115-113, 115-113, 115-113, at the end of the night. Pacquiao, though, evened the score when the two met in Vegas on Jan. 21, 2006, exerting severe pressure on a weight loss-depleted Morales en route to a 10th-round TKO win.
This time, the stakes are of a different scale than before. Morales comes off successive defeats, to slick mover Zahir Raheem, and Pacquiao, and if he loses in glaring fashion, there will be calls from critics that he hang up his mitts.
Morales was asked to compare and contrast the two rivalries.
"They are totally different," he said, through interpreter Ricardo Jimenez. "This fight with Pacquiao is a little more important because there is a lot at stake. That's how I've been looking at it."
And even though there are notable similarities between Morales and Pacquiao -- both grew up in less-than-plush circumstances -- the Mexican doesn't dwell on Pacquiao as anything other than someone who wants to knock his block off come Saturday.
"I respect both Barrera and Pacquiao as sportsmen," Morales said, "but a little more respect goes to Pacquiao because he gives everything in the ring, like I do."
Neither does Morales spend any time pondering the good fortune of having two willing combatants to spur him to exemplary levels of excellence.
"I respect both Barrera and Pacquiao as sportsmen, but a little more respect goes to Pacquiao, because he gives everything in the ring, like I do."
-- Erik Morales
"I don't think about that," he said. "I think fight to fight. Maybe in the future I'll look over my career. Now all I care about is fighting and what's next."
Morales implies that it is no coincidence that he is entering into the second memorable trilogy of his illustrious career, when other standouts are vainly combing the ranks for one.
"If you've seen my fights, you know I give the people what they want," he said. "Floyd Mayweather, for example, fights for himself. The fans see I give it my all, that's the big difference between me and a lot of fighters."
Michael Woods, the news editor for TheSweetScience.com, has written for ESPN The Magazine, GQ and the New York Observer.