Former world heavyweight boxing champion Muhammad Ali was such a dominating Olympic and professional boxer that Sports Illustrated magazine named him Sportsman of the Century in 1999. It was Ali's zest for life, a famous stand on principle, and his generosity outside the ring that made him the one of the most beloved living Americans.

"I am the greatest!" was the signature boast of the son of a Kentucky sign painter who took up boxing after neighborhood bullies stole his bicycle. Born Cassius Marcellus Clay Junior, he was soon known as the Louisville Lip for his taunts and homespun poems in an amateur fighting career topped by a light-heavyweight gold medal at the 1960 Summer Olympics in Rome. Clay shocked the boxing world by winning his first 19 professional fights and knocking out Sonny Liston for the world heavyweight championship in 1964. "You're an old, ugly bear," Clay said to Liston's face. To describe himself, he offered a verse:

This brash, young boxer is something to see,
And the heavyweight championship is his destiny.
This kid fights great; he's got speed and endurance.
But if you sign to fight him, increase your insurance!

A year before the Liston fight, Cassius Clay had joined the Nation of Islam, known as the Black Muslims, and afterward he announced he had changed his name to Muhammad Ali.

"I had been a Negro; I had no confidence," was all he would say on the subject. But he brimmed with self-assurance now.

Using a duck-and-weave style that he called his "float like a butterfly, sting like a bee technique," he was graceful yet ferocious in the ring. Outside it, he kept crowds and reporters laughing. Before his 1975 title fight with Joe Frazier in the Philippines, Ali pulled a toy gorilla out of his pocket. "It'll be a killa', and a thrilla', and a chilla' when I get the gorilla in Manila!" he said, to laughter and applause.

In 1974, a year before the Thrilla' in Manila bout, fans in Zaire had followed his every move surrounding what came to be called the Rumble in the Jungle with George Foreman. "Bomaye, Ali!" the crowd shouted. "Kill him, Ali" -- brutal words for perhaps the world's most famous pacifist, who, in 1967, refused U.S. military service, despite the threat of five years in prison and a hefty fine. He said at the time, "My conscience won't let me go shoot my brother, or some darker people, or some poor, hungry people in the mud for big, powerful America, and shoot them for what? They never called me 'nigger.'"

On college campuses nationwide, Ali spoke out against the Vietnam War while his case was appealed, and ultimately, the U.S. Supreme Court reversed his draft-evasion conviction.

Muhammad Ali won 56 of 61 professional fights. In his last two bouts, in 1980 and 1981, he was already showing the debilitating effects of Parkinson's disease, the brain disorder that affects muscle control. In one of his last television interviews, in 1985, the shuffle and slurred speech brought on by the baffling disease were evident.

The Louisville Lip is mostly silent today, replaced by a grinning -- some would say gullible -- man who gives millions of dollars to charities and individuals in need.

His critics say he has become a pathetic tool, escorted onstage to raise sympathy and donations by throwing a few imaginary punches. And those who disliked Ali, the playful braggart, blame him for inspiring today's self-centered, loudmouthed athletes.

But John Walter, an adjunct professor of ethnic studies at the University of Washington, calls Ali the quintessential American -- generous and caring; provocative; courageous physically and in his convictions; and boastful, but backing up his crowing with incredible deeds.

Professor Walter says Ali's influence has reached the world's humblest hamlets. "Down in Australia, among the aborigines," he notes, "these people saw him. They took an example from how he conducted his life, that he's willing to stand up for their rights, so to speak and to excel at the same time. You'll find many, many examples of people saying, 'I owe what I'm doing to this man.' That, to me, is profound."

Other words of admiration tumbled upon each other in November at the dedication of the $80-million Muhammad Ali Center in Ali's hometown of Louisville. Former president Bill Clinton said Ali -- who in 1990 negotiated the release of 15 U.S. hostages in direct negotiations with Iraqi dictator Saddam Hussein -- was a force for peace. "You proved once again that the power of example matters a lot more than the example of power," he said.

Muhammad Ali -- whom people remember for exuberant outbursts like, "It'll just be a beauuuuutiful fight. Ooh, I'll be dancing. I'll be so pretty. Ooh "-- turns 64 on Jan. 7. He once said of himself: "He talks a lot and boasts indeed of a power punch and blinding speed!"

And he asked, "How could anyone not see 'American' in the following: I am the greatest?"