This is a month to contemplate a great speech, full of great truths about the great American promise. It is a month to reflect on promises made and promises unfulfilled. It is a month, too, as Lincoln might have said, to be dedicated to the unfinished work that Martin Luther King Jr. so nobly advanced.

For it was 40 years ago this month, 18 steps from the nation’s memorial to Lincoln, that the Rev. King gave perhaps the most significant American peacetime speech of the 20th century. The “I Have a Dream” speech may also be, as Esther Bush, president of the Pittsburgh Urban League, put it in a conversation not long ago, “the most recognized speech in the country and the world.”

Recognized — but, oddly enough for an auditory icon, neither fully understood nor fully appreciated.

Because on that day, King spoke of more than his dream. And though the dream sequence, which King had used from time to time in earlier speeches, is now part of the soundtrack of American life, it takes up only 300 words of a dramatic oration that was the highlight of the historic March on Washington on Aug. 28, 1963. It serves as a coda. The rest of the speech speaks of the unfinished symphony of American life.

And so, in an anniversary month when the familiar ending of the “Dream” speech will be aired from Bangor to Bakersfield, it is worth contemplating the rest of King’s remarks. At this distance — four decades in which the principle of civil rights has been broadly accepted, leaving the details to work out — the beginning of the speech may even be more haunting than its conclusion.

King spoke a century after Lincoln’s act of emancipation, arguing that black Americans were “still sadly crippled by the manacles of segregation and the chains of discrimination,” pointing out that a hundred years after slavery “the Negro lives on a lonely island of poverty in the midst of a vast ocean of material prosperity.”

That still rings true today. “There are unmistakable signs of change,” Marc Morial, former mayor of New Orleans and the new president of the National Urban League, said in an interview. “This is a more open America, a more diverse America. There have been economic gains — but the gap between black America and white America remains where it was 40 years ago.”

That’s why the King speech resonates with us still. It is remembered as the “Dream” speech, but in truth it is the “Promissory Note” speech. Before tens of thousands of people gathered in the center of the capital, King set forth the challenge of America. What he demanded was not the fulfillment of a dream. He demanded the fulfillment of an obligation.

Listen to how he put it: So we have come here today to dramatize an appalling condition. In a sense we have come to our nation’s capital to cash a check. When the architects of our republic wrote the magnificent words of the Constitution and the Declaration of Independence, they were signing a promissory note to which every American was to fall heir.

Now, in truth, King was fudging history slightly; the Declaration of Independence is the nation’s founding document, but its promise of citizen rights has questionable legal standing. It is the Constitution and Bill of Rights, enacted more than a decade later, that set out Americans’ rights.

And yet the Declaration, with its assertion that “all men are created equal,” has unquestionable moral standing.

On that day in 1963, King charged his own country with defaulting on its most sacred promissory note, and from this distance we can see that the moral power with which he did it helped change America’s mind:

Instead of honoring this sacred obligation, America has given the Negro people a bad check which has come back marked “insufficient funds.” But we refuse to believe that the bank of justice is bankrupt. We refuse to believe that there are insufficient funds in the great vaults of opportunity of this nation. So we have come to cash this check — a check that will give us upon demand the riches of freedom and the security of justice.

Today the nation is a different place. The two leading foreign-policy advisers in the presidential administration of a Texas Republican are black. The majority leader of the Senate was forced to relinquish his position after suggesting that racial segregation had merit. The breadth of black entrepreneurship is reflected in the B.E. 100s, the lists of the top black-owned businesses in the nation compiled by Black Enterprise magazine; total employment in the magazine’s top black-owned companies grew nearly 15 percent from 2001 to 2002. There were only five black members of the House when King gave his speech some two dozen blocks west of the Capitol; today there are 37.

But in the entire history of the United States, there have been but four black senators, and two of them were elected from Mississippi during Reconstruction 12 decades ago. A black male is seven times as likely to be in prison than a white male, according to a study commissioned by the National Urban League. The struggle continues to this day; the check remains uncashed.

The King speech reminds us that when one part of the United States is impoverished, the entire nation is the poorer. Moments before he launched into his dream refrain, King sought to answer critics who wondered, even then, when he and his compatriots would be satisfied.

“No,” said Martin Luther King, giving the speech of his life five years before his life would end, “no, we are not satisfied, and we will not be satisfied until justice rolls down like waters and righteousness like a mighty stream.” When should we — black, white, all of us — be satisfied? Two-word answer: Not yet.

David Shribman is a columnist for the Pittsburgh Post Gazette.