Three decades ago, in his acceptance speech to the 1980 Republican Convention, Ronald Reagan quoted the famous words of Thomas Paine: "We have it in our power to begin the world over again." Reagan believed that he was running for office at a time of unusual plasticity, when long-settled arrangements--at home and abroad--could be fundamentally changed. And to the astonishment of skeptics, he turned out to be right.

This revolutionary premise is one of the silken threads connecting the presidencies of Reagan and Barack Obama. It is no accident that Obama chose to quote Paine in his inaugural address. Nor is it an accident that the explicit theme of today's Cairo speech was "A New Beginning."

As President Obama moved into his peroration, he declared,
 

I know there are many, Muslim and non-Muslim, who question whether we can forge this new beginning. Some are eager to stoke the flames of division and to stand in the way of progress. Some suggest that it isn't worth the effort, that we are fated to disagree and civilizations are doomed to clash.

Many more are simply skeptical that real change can occur. There is so much fear, so much mistrust that has built up over the years. But if we choose to bound by the past, we will never move forward. And I want to particularly say this to young people of every faith in every country: You more than anyone have the ability to reimagine this world, to remake this world.

And in his conclusion, he returned once more to this theme: "We have the power to make the world we seek, but only if we have the courage to make a new beginning."

That is the question: Do we live in a time when, through acts of imagination and courage, we can set the past aside and begin anew? Yes, the Berlin Wall fell and the Soviet Union vanished--an ironically Marxist moment when all that was solid melted into air. But not all times are like that. Barack Obama has wagered his presidency on the premise that now is another such hinge of history. If he is right, he will be a transformative president of historic stature. If he is wrong, many hopes--not just his--will be dashed, with incalculable consequences for the future.

Barack Obama is president today because he understood the moment better than anyone else and had the imagination and courage to seize it. He is conducting his presidency on the same basis as he did his campaign, and he may be right again. Whatever happens, it will be one heck of a ride.