Muhammad Ali boxed professionally from 1960, when he outscored a journeyman (and police officer) named Tunney Hunsaker, until 1981, when Ali, damaged and diminished, was beaten, in the Bahamas, by Trevor Berbick. Everyone now agrees that Ali fought too long, but not everyone can agree on how much too long. Perhaps he should have retired in 1975, after beating Joe Frazier in “The Thrilla in Manila.” Or in 1974, after beating George Foreman in “The Rumble in the Jungle.” Or in 1973, after getting his revenge against Ken Norton, who had broken his jaw earlier in the year. Thomas Hauser, a boxing historian, sometimes wonders whether Ali should have retired even earlier—during the three years, from 1967 to 1970, when Ali was effectively banned from boxing because of his refusal to join the military. “Suppose he had never come back,” Hauser told me, in a recent conversation. “We would have been deprived of all those fights, but Muhammad Ali would still be healthy today, and he would be this gleaming symbol of an absolutely unconquerable fighter.”

Manny Pacquiao is probably the modern boxer most often compared to Ali: an exciting and popular fighter who is now forty, and who has entered the part of his career during which fans can’t decide whether they want to see him retire, watch him fight, or both. It has been four years since Pacquiao was thoroughly outboxed by Floyd Mayweather, Jr., but Pacquiao remains near the top of his sport—a Top Five welterweight, most experts agree. This Saturday night in Las Vegas, on pay-per-view, Pacquiao is fighting another top welterweight, Keith Thurman, who is undefeated, and ranked about as highly as Pacquiao. The odds are close: Pacquiao opened as a slight underdog, but he is now viewed as a slight favorite, perhaps because he has so many fans willing to bet on him. On paper, this is the most intriguing Pacquiao matchup in over a decade. But it may also be a terrible mistake.

Pacquiao was an excellent and furious fighter for many years before the night, in 2008, when he beat up Oscar de la Hoya, sending him into retirement and establishing himself as a crossover star. The next year, when he demolished Miguel Cotto, Pacquiao looked sensational, and the combination of his aggressive style and his smiley, soft-spoken interviews captured the imagination of many people who hadn’t followed boxing recently, or ever.

In retrospect, though, it seems clear that Pacquiao’s career since then has been rather anticlimactic. He won a number of predictable victories over boxers who were not quite young enough, or not quite good enough. With the exception of the Mayweather defeat, Pacquiao has been the favorite in every one of his seventeen fights since 2009. Precisely one of those fights went shockingly off-script: Pacquiao’s brutal knockout loss, in 2012, to his great rival, Juan Manuel Márquez. In two other fights, referees named Pacquiao the loser: once, absurdly, in 2012, against Timothy Bradley, and another time, less absurdly (though probably still not accurately), in 2017, against Jeff Horn. Over all, Pacquiao has provided relatively little drama in the ring in the past decade, especially considering his reputation, once well deserved, for excitement.

This is not entirely Pacquiao’s fault. If he has not seemed, as Ali once did, “unconquerable,” he has nevertheless been dominant, showing himself capable of beating any boxer his size besides Mayweather. Against Thurman, though, Pacquiao doesn’t seem so formidable, and the experts—like the oddsmakers—are uncharacteristically skeptical of him. A recent poll, conducted by the boxing magazine The Ring, found an even split: eleven respondents picked Pacquiao, and eleven picked Thurman.

For half a dozen years, Thurman has been presented to the boxing public as the next great welterweight, but the plausibility of this presentation has varied. He liked to call himself Keith (One Time) Thurman, in tribute to his own punching power—he was, he liked to think, the kind of boxer who hit his opponent one time and put him on the mat. In fact, Thurman often achieved knockouts through accumulations of punches, and he hasn’t knocked anyone out since 2014; he also fought so infrequently that some fans renamed him Keith (Some Time) Thurman. His two most impressive victories, against Danny Garcia and Shawn Porter, were exceedingly close decisions, and so was his most recent fight, against Josesito Lopez, whom Thurman probably should have been able to knock out. (Thurman was returning after elbow and hand injuries, which interrupted his career for nearly two years; after the fight, he suggested that he hadn’t fully recovered yet.) Even so, Thurman remains undefeated. In Pacquiao, he is facing what every boxer wants: a big-name opponent who seems to be vulnerable. If Thurman is in an aggressive mood on Saturday night, he may well find that Pacquiao is there to be hit. He says he will be betting on himself to win by knockout.

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This, of course, is precisely the outcome that many of Pacquiao’s fans and friends would prefer not to see. Bob Arum, Pacquiao’s former promoter, recently said that he worried about Pacquiao suffering “brain damage,” and added, “I am concerned, as I would for any fighter, that when they get to a certain age, they probably shouldn’t be fighting anymore.” (Not long ago, when Arum was still promoting Pacquiao, he sounded notably more upbeat about Pacquiao’s career. “Manny doesn’t want to retire,” Arum said, in 2017, adding that he expected Pacquiao’s next fight to be “very good.”) It is true, of course, that Pacquiao is risking brain damage on Saturday night—but, then, so is Thurman, and so is everyone who boxes for a living, although in the course of a boxing career that “risk” is so high that it might more accurately be described as a certainty.

Like all athletes, boxers tend to be reluctant to stop doing something that they are, or were, so good at, even though the price they pay for not stopping can be extraordinarily high. And not all boxers feel they can afford to quit. Pacquiao is known for his generosity—he once estimated that he had spent half of his earnings on providing homes, medical care, and scholarships for poor people in his home country, the Philippines. Pacquiao has reportedly struggled, too, with tax liabilities. Unlike many aging fighters, Pacquiao has already begun a prominent second act: he is a senator in the Philippines, and an ally of President Rodrigo Duterte, who has also said he would like Pacquiao to retire. But Pacquiao doesn’t seem interested, and he says that he would be willing to fight Errol Spence, who is widely regarded as the world’s best welterweight; that fight would likely be even more dangerous for him than this one.

Boxers often get annoyed when fans beg them to stop. And they are under no obligation to listen to us, especially when some of the same people expressing those worries are also willing to watch, and pay (something like seventy-five dollars, in this case), if they choose not to retire. Pacquiao, like every fighter, must judge the risk and reward for himself, and it’s hard to say for sure how he should balance the two. What might he gain on Saturday night, besides a big fee? And what might it cost him? A decisive victory on Saturday would be a defining moment for Pacquiao’s career, and it would probably put him back on the list of the world’s greatest over-all boxers—the so-called pound-for-pound list, where he was a fixture from 2003 until 2017. But a decisive Pacquiao loss might be hard to watch, even for those backing Thurman.

No doubt many boxing fans are genuinely worried about the athletes’ health, but some of us probably harbor a more selfish concern: that we be spared, somehow, from feeling complicit in whatever suffering is caused by all that fighting. Sometimes, though, the most gripping fights occur nearer the end of a great boxer’s career—Ali, for example, slower and more broken than he used to be, staggering around the ring with Frazier and Foreman, looking beatable and sometimes winning anyway, creating a series of indelible moments and images. Nowadays, when we celebrate Ali’s legacy, we think first of those brutal encounters. Maybe Saturday’s fight will be, similarly, a late-career highlight—Pacquiao’s most dramatic performance in ages. Maybe it will provide more reason to think that Pacqiuao is a diminished boxer, one who should have retired years ago. Maybe both.