Perhaps the most famous story about Ali is also the most disputed - and that in itself is typical Ali, a self-made marketer. When he won the light-heavyweight title at the 1960 Rome Olympics, Ali wore his gold medal everywhere - even to a US restaurant where a bigoted owner said: "I don't give a damn who he is. I done told you, we don't serve no niggers." In his book The Greatest, Ali tells how he pulled from his pocket a list of 10 millionaires who had undertaken to back his career and had told him to get in touch if he ever struck problems.
He couldn't do it. "To call seemed to me to be exchanging one owner for another." In the car park, as they left, Ali and his friend Ronnie King were accosted by motorcycle gang members, who demanded Ali's medal. A bloody fight ensued after which Ali and King were washing themselves in the Ohio River and, when they returned to a bridge over the river, Ali allegedly threw the medal in.
"The medal was gone but ... I felt calmly relaxed, confident. My holiday as a White Hope was over. I felt a new, secret strength."
No-one knows whether the story was true but he was awarded a replacement medal at the 1996 Atlanta Olympics and that's the sum and substance of it - Ali's strength of character, humour and fame combined to cement the legend, no matter whether its beginnings were muddied in half-truth or not.
He was hated when he stepped up against the imposing Sonny Liston, with most fight fans hoping the "big ugly bear" would knock the block off the brash young hopeful. In the first fight, a blazing Ali used his speed and lightning combinations to unsettle Liston, who seemed out of condition and frustrated by the young challenger. Liston failed to answer the bell for the seventh round and Clay was declared the winner by technical knockout - one of the biggest upsets in boxing history.
There has been speculation about whether Liston's claimed shoulder injury was severe enough to prevent him from continuing the fight. There were reports that the injury was false - an excuse to trigger the re-match clause in the contract.
The second fight, in 1965, was even more controversial. Midway through the first, Liston fell in what many argued was not a legitimate knockdown.
The blow that ended the match became known as "the phantom punch," so named because most people at ringside did not see it or weren't even sure it had been delivered. Even Ali was unsure whether the punch connected, as footage shows Ali asking his entourage "Did I hit him?" after the bout.
Suspicions of a fix persisted and Liston certainly mixed in mafia circles; being a runner and a small-time dope dealer for them. He was found dead in 1970 surrounded by a glass of vodka, heroin, a gun and a crucifix. The official verdict was suicide through intravenous heroin - even though Liston had a well-documented phobia when it came to needles. Rumours of a mob hit continue to this day. There were claims Liston had bet against himself and "took a dive" - because he owed money to the mafia.
Once past Liston, Ali never looked back. But that was the beginnings of the greatest sportsman ever - surrounded by racial and religious animosity, crime, opponents with dubious records and character - a man alone, but alone with his talent.
Perhaps the best way to mark the champ's 70th is to watch that marvellous documentary Facing Ali, where 10 of his top opponents talk about him. Three - Frazier, Ron Lyle and Henry Cooper - have all died in recent months; Ali outlasted them in that fight too.
But the three play their part in a compelling look at the nobility and tragedy of boxing. If you want to see Ali in his pomp, watch the amazing doco When We Were Kings - the tale of how he overcame Foreman and unforgettably, irretrievably, annexed Africa to the Ali legend.
Watch them, rather than the shaky, debilitated man he has become. And yet, there is still a strange kind of majesty to the man - his birthday celebrations are raising money for research into Parkinson's and other health issues.
It was always thus with Ali. He was the champ, sure, but it was all the other issues that surrounded him and which he also KO'd that really made him the greatest.