It is the core American faith - that this is a country uniquely, even Providentially blessed, whose best days always lie ahead of us.
And for much of our history, that faith has been justified by reality. Throughout most of the 20th century - the "American Century," as LIFE Magazine proclaimed it - America WAS the richest, most powerful, most open society on Earth.
But today, a specter is haunting America: A sense that we will no longer be THE dominant world power.
And, more troubling, a sense that our future may NOT be brighter than our past, a sense captured in a recent poll revealing that nearly half of Americans believe our best days are behind us.
In a way, this is nothing new, says author and journalist James Fallows.
"As the early American republic was first taking form, already there were warnings - 'Are we going to become Rome?' - even before there was any sort of great empire to worry about," said Fallows.
And through the last half-century in particular, we worried about threats to America's supremacy.
"I can remember the launch of Sputnik in the late 1950s, and the sort of galvanizing effect that had on American education, especially in science and math," said Fallows. "And then, in more recent history, say, starting 25 years ago, was the Japanese. And, of course, in the recent decade it's mainly been China that's been the comparison by which America is falling short."
This idea - call it "relative decline," the concept that other nations may equal or surpass our economic or technical mastery - may be hard for a "We're Number One!" America to accept. But for Paul Kennedy, who wrote "The Rise and Fall of the Great Powers," it is nothing new - nor troublesome.
"No one stays on top," he said, "and the U.S. has a very hard time in realizing that. Great powers, like the U.S. - like the British Empire, like the Ottomans - rise to a position of prominence over, you know, a century and a half or so, and stay at the top for a long while, and then, usually, gradually decline."
But it's a different idea that you can sense in the public conversation today: A sense that the some of America's most enduring beliefs about itself are in doubt . . . beliefs that are at the center of what kind of country we are and how effective our political system is in dealing with what threatens us.
At the center of the doubts is the wounded American economy: Not just the 15 million who are jobless or underemployed, not just the trillions lost in the economic meltdown of the last few years, not just the millions who have or may soon lose their homes.
It's the broader picture:
"In the last 35 years, the median wage in America has gone down," said Fallows. "Most Americans, in the last, now, two generations, have done worse economically, which is unprecedented in American history."
And it poses a special threat to a pattern we took for granted from World War II through the '70s - blue-collar workers finding secure well-paying jobs that provided them and their families a measure of security and comfort.
Moreover, if America's economic machinery is stalled, it means trouble - serious, potentially devastating trouble - for our governments, and their obligations.
Our federal government has a $14 trillion debt. And in towns, cities, counties and states, pensions and health care costs of public employees threaten to leave those governments literally bankrupt.
Former New York lieutenant governor Richard Ravitch has spent a lifetime dealing with budget crises. This time he says it's different - and worse.
"It is not sustainable - there is no question about that," Ravitch said. "And I think that it's only a matter of time before you begin to see bad things happening, like cities or counties not making payroll, defaulting on debt."
So, is optimism a thing of the past? - The answer (to James Fallows) is yes AND no.
"We could solve what's wrong with this country fairly easily, considering how rich we are, all the resources we have," he said. "Whether we'll actually do that, is a different matter."
The real question may be: Is our 200-year-old political system capable of dealing with today's dilemmas?
"I think we have a very poor constitutional and political system for the 21st century," said Kennedy. "We have a system which was marvelous for 13 independent, loosely-tied states in 1783, 1786," he laughed.
"I've spent a lot of years of my adult life living outside the United States," said Fallows. "People will often say, 'I wish we had X, Y, and Z, in comparison with the United States.' No one has ever said to me, 'I wish we had your U.S. Senate. I wish we had some governing system like yours,' because the ability of our political system to match our great potential resources with the problems we have, that seems different to me from times I know about before."
Which is, says historian Paul Kennedy, why America's most urgent need it to distinguish between what is beyond our reach . . . and what is in our power to change.
"The reversible things are what you do with your national budgets, with your science and technology programs, with your education," said Kennedy. "Those are reversible.
"And, therefore, my answer to you about, you know, is America ultimately in decline is that there's the irreversible stuff. Just forget about it. Concentrate on what IS improvable, what IS reversible - and then we start looking better."