Muhammad Ali's conscientious objection to the War in Vietnam is the first social/political issue I can remember capturing my attention. When Ali refused induction into the military in 1967 I was 12 years old. My family had just returned the previous year from Germany where my dad the Army sergeant had been assigned. We had had no television to speak of in the small Bavarian town of Bad Kissingen, so the ferment of the civil rights movement, for example, didn't penetrate my consciousness. (I remember when we landed in the United States from Germany being transfixed by an American television show--a black and white episode of Lost in Space featuring Billy Mumy--broadcast somewhere in the airport.)
Everything about the Muhammad Ali case confused me. 
Of course most people then were still calling him Cassius Clay, including my parents. My father had no sympathy for Ali's refusal to go to Vietnam and yet he had admired the brash irreverence of Cassius Clay the boxer. I remember wondering why such an attractive person would risk all that success by making an unpopular argument. I couldn't imagine anything ever being so important. And yet I also remember disagreeing with Ali's critics who questioned his patriotism and manhood. The one thing he didn't seem to lack was courage.
Fifty years later, Carmen the adult-approaching-senior-citizen has achieved more clarity about the example of Muhammad Ali. In a wonderful retrospective I recommend to all Rebels at Work, Ali is quoted as saying during the height of the controversy:
I have nothing to lose by standing up for my beliefs.
Actually, he had just about everything to lose materially. Because of his decision, Ali was stripped of his heavyweight boxing title--the most prestigious athletic honor of that era--and was unable to fight during what should have been his most productive years. He lost a lot. But, as the article makes clear, Ali's principled stand buttressed others to do what they thought was right, including female tennis star Billie Jean King and Nelson Mandela, who, it should be remembered, was a heavyweight boxer himself in 1950s South Africa.
I think Muhammad Ali intuited the impact that a single individual can have when he stands for something beyond just himself. He took on the most extreme of positions at the most inopportune of times and was ready to suffer the consequences if proven wrong. He understood what a 12-year old couldn't and what many adults still don't:
Life's ultimate success is being true to yourself.