Nearly 50 years ago, when I arrived in Minnesota as a student fresh from Africa, I had much to learn -- starting with the fact that there is nothing wimpish about wearing earmuffs when it is 15 degrees below zero. All my life since has been a learning experience. Now I want to pass on five lessons I have learned during 10 years as secretary general of the United Nations that I believe the community of nations needs to learn as it confronts the challenges of the 21st century.

First, in today's world we are all responsible for each other's security. Against such threats as nuclear proliferation, climate change, global pandemics or terrorists operating from safe havens in failed states, no nation can make itself secure by seeking supremacy over all others. Only by working to make each other secure can we hope to achieve lasting security for ourselves. This responsibility includes our shared responsibility to protect people from genocide, war crimes, ethnic cleansing and crimes against humanity. That was accepted by all nations at last year's U.N. summit. But when we look at the murder, rape and starvation still being inflicted on the people of Darfur, we realize that such doctrines remain pure rhetoric unless those with the power to intervene effectively -- by exerting political, economic or, in the last resort, military muscle -- are prepared to take the lead. It also includes a responsibility to future generations to preserve resources that belong to them as well as to us. Every day that we do nothing, or too little, to prevent climate change imposes higher costs on our children.

Second, we are also responsible for each other's welfare. Without a measure of solidarity, no society can be truly stable. It is not realistic to think that some people can go on deriving great benefits from globalization while billions of others are left in, or thrown into, abject poverty. We have to give all our fellow human beings at least a chance to share in our prosperity.

Third, both security and prosperity depend on respect for human rights and the rule of law. Throughout history human life has been enriched by diversity, and different communities have learned from each other. But if our communities are to live in peace we must stress also what unites us: our common humanity and the need for our human dignity and rights to be protected by law.

That is vital for development, too. Both foreigners and a country's own citizens are more likely to invest when their basic rights are protected and they know they will be fairly treated under the law. Policies that genuinely favor development are more likely to be adopted if the people most in need of development can make their voice heard. States need to play by the rules toward each other, as well. No community suffers from too much rule of law; many suffer from too little -- and the international community is among them.

My fourth lesson, therefore, is that governments must be accountable for their actions, in the international as well as the domestic arena. Every state owes some account to other states on which its actions have a decisive impact. As things stand, poor and weak states are easily held to account, because they need foreign aid. But large and powerful states, whose actions have the greatest impact on others, can be constrained only by their own people.

That gives the people and institutions of powerful states a special responsibility to take account of global views and interests. And today they need to take into account also what we call "non-state actors." States can no longer -- if they ever could -- confront global challenges alone. Increasingly, they need help from the myriad types of association in which people come together voluntarily, to profit or to think about, and change, the world.

How can states hold each other to account? Only through multilateral institutions. So my final lesson is that those institutions must be organized in a fair and democratic way, giving the poor and the weak some influence over the actions of the rich and the strong.

Developing countries should have a stronger voice in international financial institutions, whose decisions can mean life or death for their people. New permanent or long-term members should be added to the U.N. Security Council, whose current membership reflects the reality of 1945, not of today.

No less important, all the Security Council's members must accept the responsibility that comes with their privilege. The council is not a stage for acting out national interests. It is the management committee of our fledgling global security system.

More than ever, Americans, like the rest of humanity, need a functioning global system. Experience has shown, time and again, that the system works poorly when the United States remains aloof but it functions much better when there is farsighted U.S. leadership.

That gives American leaders of today and tomorrow a great responsibility. The American people must see that they live up to it.

The writer, secretary general of the United Nations, will leave office Dec. 31. This article is based on an address he will give today at the Truman Presidential Museum & Library in Independence, Mo.