J. William Fulbright is 80 now, still hard at work in his downtown Washington law office and still protesting, incorrectly, that his views on world events are not worth considering given his age and his years away from public service in the Senate.

"At that time," he was saying Thursday, reflecting on his feelings exactly 40 years ago that very day when the United Nations was born, "I was still young and vigorous and hopeful about the world, and I thought surely we would take the lessons about the atomic bombs and the war and all that had happened seriously. But it's just incredible how slow we are to learn the facts of life."

It was Fulbright's resolution, introduced when he was a freshman congressman from Arkansas in the spring of 1943 during the grimmest days of World War II, that paved the way for participation in a postwar international organization, reversing the U.S. policy of isolationism that followed the tragedies of this century's first world war. His resolution was only one sentence long:

"Resolved: That the House of Representatives hereby expresses itself as favoring the creation of appropriate international machinery with power adequate to prevent future aggression and to maintain lasting peace, and as favoring participation by the United States therein."

Viewed from the perspective of two generations later, it's easy to see why President Reagan characterized the emotions of 40 years ago that led to the U.N. founding as "innocent dreams" stemming from a world yearning "to believe in ideals with innocent trust" when he delivered his anniversary address to the U.N. General Assembly.

Certainly the United Nations has failed to fulfill the hopes expressed for it. It has not prevented aggression. It has not maintained lasting peace. It has not stopped the proliferation of nuclear weapons. In the years since the end of the wholesale slaughter that was World War II, more than 100 new wars have been fought and some 10 million lives lost, the nuclear arms race has spiraled ever upward and the United Nations has demonstrated what its harshest critics are quick to point out: that it has degenerated into a fractious, posturing, ideologically divided and largely impotent body.

That record makes all the more poignant the emotions recalled last week by Fulbright and others who were there at the creation. But there's a lesson in them, too. The United Nations wasn't created out of naivete, as the president suggests. It stemmed from recognition of a hard reality: There was no alternative but to try. And it was spawned by a determination that future leaders would not repeat the bloody mistakes of the past.

Thus, the words of the Texan, George C. Mahon, when the Fulbright resolution reached the floor of Congress:

"Mr. Speaker, this is a historic day in the history of our country. Today we propose to pass a resolution which serves notice to the world that when we have won this war we shall try to keep it won. Never before has such action been taken by Congress."

Thus, the most dramatic moment in the ensuing debate when the ranking Republican member of the House Foreign Affairs Committee, white-haired Charles A. (Doc) Eaton of Ohio, stood to say passionately:

"I have seven sons. One died of war wounds July 30. Another was killed over Nuremberg Aug. 10. By God, I will vote for any measure which carries some hope of the future abolishment of war's wholesale murder."

On the occasion of this anniversary, the last in the series of observances of great and tragic events that happened 40 years ago and shaped the world in which we live today, there is no need to reexamine the record of mistakes and missed opportunities and dashed hopes that are the history of the United Nations. Everyone, everywhere, knows them.

Everyone also knows the record of deceit and brutal power politics employed by the Soviet-led eastern bloc, a record in which our side has sadly shared, especially of late.

During the Reagan years, the United States in a number of ways has dissociated itself from the United Nations as an institution and signaled an intent to go it alone. It has pulled out of UNESCO, withdrawn from the World Court, cast the only vote against a World Health Organization code regulating the sale of infant formula, refused to sign the Law of the Sea treaty, withheld its funds for family-planning and population-control programs. Its U.N. representatives continually have publicly disparaged and denigrated the organization and even, two years ago, invited the members to remove themselves from U.S. soil with the taunting comment from the American emissary that if they did so, "We will be at dockside bidding you a fond farewell as you set off into the sunset."

Someone like Fulbright sees this as evidence of lack of a sense of history.

"The U.N. is just machinery," he says. "It reflects the attitudes of both superpowers, and it cannot be any better than its two principal actors. I'm sorry to say that we just think it's not important. We say we are a nation of laws, but we don't want to have anything to do with the World Court, for instance. The U.N. is still useful as a forum for education and for the forming of public opinion, but it cannot execute its main function with regard to peace. It lacks the power to do so because the two great countries will not accept the necessity of transferring their power and accepting the decisions of a supreme authority. We won't submit anything. We're a bull in the china shop. We go our own way. We take the attitude that we're always right, and we don't even tolerate suggestions to the contrary. It all strikes me as another example of the arrogance of power."

There is another possibility: that Ronald Reagan's words last week before the United Nations about sustaining and celebrating life and advancing the cause of peace mark a more hopeful turn. In the last 40 years, few have expressed these old aspirations better or with more eloquence. Whether it's rhetoric or a new recognition of reality on the part of the president cannot be judged now. At least it's not a rejection of the reasons why the United Nations was created. Reagan has memorably affirmed that the need for international cooperation still exists.