It is better to be a half-formed and rough idea than a brilliant cliché. Such preference for the imperfect new defines America. As we Americans celebrate the birth of our nation, can we put words on the reason we love it? Let me try.

Because Europeans measured what they found here against what they had left behind, newness was the main note of the settled land. In the beginning, religiously enflamed politics had made life intolerable in the old country, a story that achieved its master form with the coming to Virginia and Massachusetts of the English dissidents.

But even the mythic 1492 had carried an implication of the New World's liberating significance, for in addition to sponsoring Christopher Columbus, the monarchs Ferdinand and Isabella chose that year to expel Jews and Muslims from Spain, establishing the totalitarian principle in Europe.

Even as Spaniards then wreaked purposeful and accidental havoc in the New World, they opened an unforeseen escape route from the old.

America, for all of its nascent idealism, began as an instance of brutal European imperialism, with the exterminating of indigenous peoples and the enslavement of Africans as essential elements.

But because that nascent idealism found articulation in the solemn compacts of the early generations - culminating first in the Declaration of Independence that we commemorate on Tuesday, then in the U.S. Constitution, then in the Bill of Rights - American imperialism contained principles of its own self-criticism. Slavery came to be seen as an abomination less in contrast to the practice of other nations than to the establishing theory of this one.

America began, that is, as a half-formed and rough idea, but that idea became the meaning against which all life in the United States has been measured ever since. That idea has been a perpetual source of newness, even as it has become more fully formed and clearly articulated.

And what is that idea? It comes to us by now as the brilliant cliché of the Fourth of July, but with stark simplicity it still defines the ground of our being: "All men are created equal."

That the idea is dynamic, propelling a permanent social transformation, is evident even in the way that word "men" strikes the ear as anachronistic now.

That Jefferson and the others were not thinking of women matters less than the fact that they established a principle that made the full inclusion of women inevitable. And so with those who owned no property, and those who were themselves owned property.

How new is this idea today? Its transforming work continues all around us.

Last week, the U.S. Supreme Court faulted the Bush administration for its treatment of detainees in Guantánamo, implicitly affirming that one need not be a U.S. citizen to claim basic rights. The foundational principle extends to enemy combatants. They, too, are created equal.

And so in other areas. U.S. politics is obsessed with the question of the place of immigrants, legal and illegal. The mainstream argument takes for granted that even here liberalizing change is under way. Confronted with an "illegal" person, the law must still give primacy to personhood. And, on another front, is it an accident that American Episcopalians are the ones challenging the world Anglican body on the question of equality for gays and lesbians?

America is by definition unfinished, because it forever falls short of itself. Not that the United States is more moral than others, but its half-formed foundational ideal required a moral purpose at the start - and a moral purpose to the end.

That is both creative and creatively undermining. Born in a challenge to authority, American authority continually inhibits its own exercise (what the Supreme Court did last week in challenging the executive and legislative branches over Guantánamo). Recognitions of personal alienation inevitably open into demands for the reform of alienating systems - and in America that is the work of politics. It never stops.

Contention is essential to such a social dynamic. Much as the polarized character of national life is bemoaned, the red state/blue state acrimony reveals the genius of what the founders began, for the structures of this public order evolve within a framework that continually transforms conflict into energy for change.

The irony, of course, is that those who declare their loyalty to the brilliant cliché of an unchanging past are themselves at the service of the imperfect new. After all, to be an American traditionalist - and isn't this what Americans celebrate on July 4? - is to affirm the revolution.

James Carroll: What we love about America - IHT