His decades in prison could have destroyed his love of freedom, but they nourished it instead.

I am writing this column on my birthday. It is a significant birthday for me. Without getting too specific, let me just say that Mick Jagger and I are now the same age, Mick having achieved this milestone last week.

At this point in my life, I find myself reflecting on my heroes – those whose lives have been an inspiration to me. Foremost on this list is Nelson Mandela, who turned 90 two weeks ago.

Most know the outline of Mandela's remarkable life. Born in South Africa at a time when there were few opportunities for blacks, he found a way to get a university education.

He became one of the first black lawyers in South Africa in the 1940s and began his lifelong quest for greater equality for his people.

The most remarkable aspect of his life is that he spent most of the prime of it in jail – from the mid-'60s until his release in 1990. In 1994 he became the first democratically elected president of South Africa.

All of this is chronicled in his memoir, "Long Walk to Freedom." This is one of those rare long books that is so riveting that it is difficult to put down – start to finish.

In it, Mandela recounts his years in the notorious maximum security prison on Robben Island.

The years were made bearable first by his refusal to accept the harsh and degrading early tactics of his captors, and then by his leadership in turning the prison into a learning center where the ANC leadership studied history and politics in a make-shift university.

Through it all, he was steadfast in refusing all offers of pardon or release unless they were unconditional.

Then came a highly unusual penultimate chapter to his captivity, when some of his warders took him on clandestine visits to "town" to acclimate him to modern South African life.

In fact, one of the more moving aspects of his release was his farewell visit of thanks to one of his white warders, whose family had become friends.

I am fortunate in that I was able to meet Mandela and hear him speak when I was on a visit to Capetown in January 2003. President Mandela addressed a dinner to inaugurate a new program to build schools in rural South Africa.

As you might expect, his presence fills a room. He radiates a warmth and dignity that is at once reassuring, yet almost regal. That evening he put aside his text and spoke from the heart about his and his people's long journey to freedom.

It was inspiring. At that time he was, of course, in his mid-80s, but he had lost none of the grace, humanity, and determination that mark him as one of the 20th century's great leaders. Even in retirement, Mandela still provides much of the moral force for a multiracial society in South Africa.

South Africa has so far been one of the few places in Africa where a multiracial society flourishes. Will it outlive Mandela? Most believe this will depend on the quality of leadership that follows the current president, Thabo Mbeki. South Africa remains a fragile society.

Meanwhile, those of us in the rest of the world can take heart from the lessons of Nelson Mandela. He has been a leader who refused to let the harshness of apartheid and of his own circumstances dim his belief in the ultimate humanity of man.

He was a major force in one of the most remarkable transitions of a society that we have witnessed in the past 100 years.

So I raise a salute to Nelson Mandela at 90. Whenever I am discouraged by the challenges that America and Maine face, I take heart in Mandela's closing comment in his book:

"I have walked that long road to freedom. I have tried not to falter. I have made miss-steps on the way. But I have discovered the secret.

"After climbing a great hill, one only finds that there are more hills to climb. I have taken a moment to rest, to steal a view of the glorious vista that surrounds me, but I can only rest for a moment for with freedom comes responsibilities, and I dare not linger for my long walk has not ended."