Mickey Rourke is an actor, not a boxer. He proved that the hard way. Those who saw him fight say he was pretty good, all things considered. Those of us who thought they saw a movie star being born back in the 1980s can only look at Rourke now and wonder: what the hell happened?

It isn't complicated. Certain recreational activities aside, he got his head punched in. Thanks to a brave, Quixotic desire to prove something to himself as a fighter, a face that was once startlingly handsome is now a weird, puffy, gargoyle mask.

That fact, plus a talent that refused to die, gives The Wrestler, Rourke's report from the lower depths of so-called sport, much of its poignancy. You could read the movie in any number of ways, but the core narrative is simple. This is what happens, ever more dangerous, always tragic, to those who risk full-contact contests and refuse to quit.

That story is as old as boxing. It forms the long, dreadful coda to the career of the greatest fighter of them all. There are some, idiots all, who still like to claim that Muhammed Ali might have been afflicted by Parkinson's had he never entered the ring. Anyone who watched him through to the end, when he seemed to take a perverse pleasure in the suffering he could absorb, knows better.

The knowledge perhaps explains the collective guilt that underpins boxing. No one forced Ali into those comebacks, we say. No one made him take those appalling risks. But who provided the audiences and the purses? We did, of course. Then we lamented the awful decline we had procured.

Watch any professional fight and you are complicit. That's the truth, but it is also the heart of a paradox. Doctors have explained, time and again, what happens to a boxer's brain when he takes a punch: it sloshes around, literally, inside the skull. Repeat the process too often, or simply apply it to an especially unlucky individual, and tragedy ensues.

So should we ban boxing? That's the logical answer to a sport that has nothing to do with logic. Should we insist on the rules and protections applied in the amateur game? The last Olympics saw some impressive fights, but be honest: it wasn't the same. Hence the paradox: no one wants anyone to get hurt, yet someone has to get hurt. And a ban that drove boxing underground would merely create bloody human cock fights.

The sport has a strange way of lowering the spirits and raising them simultaneously. I can think of few things more depressing than the fact that Mike Tyson and Evander Holyfield will engage in a "rematch" this year in Abu Dhabi. Yet at the same time there is nothing more cheering, even inspiring, than the dignity, grace and humanity of Joe Calzaghe. Nothing becomes his career so much as the manner of its ending.

Given the career, that's saying something. So what if he is three victories short of Rocky Marciano's 49-bout record? Sixteen years undefeated, 11 of those as world champion, is a monumental achievement. At 36, the Welshman has done it all.

He could have a rematch with Roy Jones or Bernard Hopkins; he could make a lot more money. But what if he lost, finally, to the likes of Kelly Pavlik? How much cash would it take to heal that scar?

Last week, with his usual honesty, Calzaghe did not pretend that he has not been tempted. He has agonised over his decision, in fact, since his points win over Jones in New York last November. But his definitive statement rings true: "I've come to the point where the satisfaction of retiring undefeated has to outweigh the thrill of another fight."

Why do so few share that insight? Now Calzaghe can never be beaten. Now he is beyond the risk of harm. He has unified the super-middleweight titles, the first man to do so. He has seen off all-comers in that division and as a light-heavyweight. He will be a champ-ion, undisputed, for the rest of his life. And his family will not have to live with the pain of witnessing a supreme athlete risk becoming a shambling wreck.

His children wanted him to quit. His mother asked him to quit. Calzaghe listened to them, not to the promoters or any of the other sub-species who cluster around the honey pots of boxing. It speaks better of the man than any of the statistics, remarkable as those are. It seems to tell us, against every instinct, that the sport still allows room for some humanity.

Calzaghe retires with money, mem-ories and pride. As long ago as 1997 he scared Steve Collins out of the WBO super-middleweight title and then fought Chris Eubank - no joke in the ring, at least - to a standstill. Jeff Lacy was undefeated and the latest great American hope in Manchester in 2006. Calzaghe cut him to pieces, almost literally, in a technically-perfect blur.

Yet the Welshman could, in boxing's most ominous phrase, always take a punch. He proved that in front of 50,000 people in Cardiff in 2007. The Dane Mikkel Kessler could have used his fists to drive nails, had he been so inclined, but Calzaghe took the blows. He then proceeded to outbox a fighter who that night would probably have beaten anyone else on the planet.

It has been a career full of such highlights. In fact, blemishes are nowhere to be found. So why sully perfection? Why become one of those athletes who allow delusive dreams to supplant reality when age begins to tell its story? Sport is full of them, the David Beckhams, the Lance Armstrongs, the - God help us - Mike Tysons. Sometimes they beat the odds. Yet somehow even when they win they lose.

Calzaghe seems to know better. I truly hope that nothing and no one causes him to change his mind. In his family he has the best answer of them all, and nothing, nothing at all, to prove.