Ali emerged at the intersection of two of America's major social dramas of the 20th century – the civil rights movement and the Vietnam war. He was also the heavyweight champion of the world during boxing's golden period, which is almost unimaginable to anyone whose estimate of the sport proceeds from its current sorry state.

When Ali – then called Cassius Clay – beat Sonny Liston to win the world title for the first time in 1964, he shouted at the ringside reporters: "Now, I'm king of the world!" In one sense, he was. At a primal level, the heavyweight champion of the world is the man who need fear no one. That was Ali's view, also. He had no fear. He had tremendous ego. He had a mythological view of himself and dared to live up to the myth. Initially, when he said he was the greatest, everyone said he had a big mouth. Now, 50 years later, everyone says he was the greatest.

He was a dazzling athlete, impacting on boxing in the way that black musicians such as Little Richard and Chuck Berry were then impacting on popular music. He was criticised for his style which paid little heed to boxing's conventions. He carried his hands low, inviting his opponent to hit him. When the inevitable barrage of punches came his way, he just stood his ground and, relying purely on his reflexes, bobbed about while blows whistled to the right and the left of him. Sometimes in the ring, he danced mesmerising circles and half-circles, hitting his opponents, who had never dealt with anything like him, at will.

I was eight when he beat Liston. It was before television, but I still remember hearing some sort of explosion from the other side of the world. Then came the rematch and Liston lay on the canvas refusing to fight. The famous photo shows Ali, this peerless young warrior, screaming down at him.

No one believed he could beat Liston the first time. He destroyed him. The following day, he announced to the proudly christian United States of America that he was a Muslim. He took the name Muhammad Ali shortly afterwards.

And that's another reason I don't think there will ever be another Muhammad Ali. He also stood at the intersection of two other great historical forces – Islam and the United States.

To some extent, the two merged – Muhammad Ali, black American Muslim warrior, was now projected to the world, and one billion Muslims worldwide, with the full force of American popular culture.

In a way that no current world sports star comes remotely close to emulating, Muhammad Ali is a major historical figure of the past 50 years. His great moment came outside the ring when, in 1967, he refused to enlist in the American army and fight in the Vietnam war. There is footage of him addressing students at white universities at the time, saying: "Kill them [the Vietnamese] for what? They never called me n-----, they never lynched me, they never set their dogs on me, they never raped my mother. If I'm gonna die, I'll die right here fighting you." He risked assassination and the hostile attention of J Edgar Hoover's FBI.

Muhammad Ali's historical legacy, including the nuance of his race politics, deserves far more detailed scrutiny than I can give them here. But what nobody can deny is the radical moral force he brought to the debate surrounding the Vietnam war by asking the simplest of questions: Why am I going to kill these people? Where is my responsibility in this? He accepted the probability of going to jail, was stripped of his world title and suffered a total loss of income. His stand was a clean, simple act unclouded by moral ambiguities and, in direct proportion to its simplicity, was its force.

This week, Rolling Stone magazine had an interesting article by Matt Taibbi headlined, "Muhammad Ali was a hero but his enemies have a legacy too". Taibbi argued that two big things have changed since Ali made his stand against the Vietnam war. The American government ended conscription in 1973. Never again would celebrities be able to highlight opposition to government policy by refusing to enlist.

The other change is the way the US governments and military now control the media coverage of military matters. Taibbi claims the Pentagon now has the biggest public relations budget of any organisation in the world. Compare, he says, the social activism which arose in opposition to the Vietnam war with the moral torpor that surrounded the disastrous invasion of Iraq in 2003.

In 1990, Muhammad Ali went to Iraq – where he was celebrated as a famous Muslim — and successfully negotiated the freedom of 15 Americans being held hostage by Saddam Hussein's regime. Ali found himself as famous in Iraq as he was in the US. Accused of doing it for publicity, Ali replied he needed publicity for his fights but not for helping people: "Then it's not sincere."

The former radical's place in mainstream American culture was confirmed when he lit the torch at the 1996 Atlanta Olympics. Maybe he was just too big a star – too American an icon – for Americans to ignore. He was also in the grip of Parkinson's disease. In 2007, I met one of Ali's daughters, Rasheda. She said her father grew "more giving" the older he got. By the end, Ali was one of humanity's elders.

Banned from boxing for three years while in his prime, he won the world heavyweight title three times. Over the course of his long career, he laid bare his enormous courage and astonishing willpower, but this was also where his story acquired the symmetry of classical mythology since those same qualities also blinded him to the punishment his mind and body took in the latter part of his career when he discovered he could exhaust opponents by letting them hit him.

Like so many great sportspeople, his career climaxed with a great opponent, Joe Frazier. While the world looked on, the pair nearly killed one another during their third bout in Manila. Frazier had plenty of reason to dislike Ali and did so with a passion until the day he died. Frazier believed that Ali used against him the sort of race politics that Ali decried in the white community, calling Frazier an "Uncle Tom" and thereby denigrating him in the eyes of black Americans. Frazier is the shadow following the Ali legend.

Ali came to Melbourne in 1979 to do an advertisement. The story goes that he said to the backer of the trip: "This used to be a black country, didn't it? How come I can't see no black people?" The backer made a few hasty inquiries and Ali was driven to the Aboriginal Health Service in Fitzroy.

Years later, I met an old taxi driver who had Ali in his cab during that visit. Ali had tipped him $20.

The old man had kept the $20 note until one day, deciding it was only money, he had given it to his granddaughter. I might have given it to my granddaughter too, but I would have said: "Keep it and remember the day your grandfather, a humble taxi driver, met the most famous man on the planet."