A press conference on the Millennium Development Goals (which were meant to be completed by the end of 2015) at the annual World Economic Forum meeting in Davos, Switzerland, in 2008. Photograph by Laurent Gillieron / Keystone / AP

In 2000, the United Nations, an organization that has often proved more effective in theory than in fact, established a series of eight goals to help the world’s poorest people. The Millennium Development Goals, as they were known, were an attempt to address the most basic requirements of human life: reducing the child-mortality rate by two-thirds, reversing the course of the AIDS epidemic, vastly increasing the number of people with regular access to safe drinking water. Other targets sought to move closer to a world without poverty or hunger, in which common diseases have been defeated—a world with universal primary education, gender equality, improved maternal health, and environmental sustainability.

Taken together, the M.D.G.s, which were meant to be completed by the end of this year, can be regarded as the largest collective promise that the world’s governments have ever made to their citizens. But while the United Nations issues a lot of declarations and holds a lot of assemblies, words are not the currency of hunger or poverty. Few people assumed that goals so lacking in controversy that they were endorsed by every member nation could possibly be fulfilled.

Depending upon how one interprets the words in the goals, progress has been mixed. We have not been able to lower maternal mortality by two-thirds. While improved drinking water has been extended to more than 2.6 billion people since 1990—a remarkable accomplishment—more than six hundred and fifty million still lack access to safe water, and nearly a billion people have no sanitation facilities at all. Inequality has deepened between men and women, between rich and poor, and between much of the developed world and the poorest countries, particularly in Africa.

Yet goals are often a combination of guesswork and dreams. And it has become too easy to make the mistake of thinking that the fact that we have failed to achieve a number written down at the turn of the century means that we ought not to have bothered. There has never been more prosperity, possibility, or promise for a higher proportion of the Earth’s population than there is today. Fewer than half as many people live in complete poverty (defined by the World Bank as surviving on less than $1.25 a day) as did twenty-five years ago.* And although we did not reach the goal of reducing childhood mortality by two-thirds, there is no country—with the possible exception of Syria—where infant or child mortality is higher than it was before the M.D.G.s were announced, in 2000. Between 1990 and 2013, vaccination rates rose dramatically, and mortality rates for children under five declined by forty-nine per cent, from an estimated ninety deaths per thousand live births to forty-six per thousand. In 1840, at the height of the British Empire, its citizens could expect to live to forty. Today, the global life expectancy is about twice that, and a child born in one of the world’s poorest nations is likely to live longer than the richest Americans lived a century ago.

Nonetheless, as the M.D.G. deadline approaches, many people have been left behind. At least a billion people go to bed hungry each night. Millions of women are treated nearly as slaves, and a horrifying number still die in childbirth.

This September, the U.N. will meet to ratify a much broader set of targets—this time called the Sustainable Development Goals—which many U.N. member states will use to frame their agendas and political policies in the next fifteen years. Officially, there are seventeen new goals—and the environment, which was all but ignored in the last round, will receive much-needed attention. But with scores of countries participating in consultations, there are now hundreds of targets, sub-targets, and ancillary targets within the over-all goals. Whether one supported the M.D.G.s or not, it is hard to deny that they provided a focal point—and some sense of shame—for governments and N.G.O.s attempting to eradicate poverty. It will be nearly impossible to focus on the S.D.G.s in the same way because, with goals such as “End poverty in all its forms everywhere,” many seem so broad that they will be easy to ignore.

“Having 1,400 targets is like having none at all, and so governments need to make some hard choices, deciding which targets will offer the greatest returns on investment,’’ Bjørn Lomborg, the president of the Copenhagen Consensus Center, has written. Lomborg convened many of the world’s leading economists to assess the best way to choose the next development goals. He argues that the U.N. is diluting its power by attempting to eliminate all problems. He is undoubtedly correct. Costs in the S.D.G. plan rarely seem to be linked to benefits. “Of course, economics alone should not determine the world’s top development aims over the next decade and a half,’’ he said. “But ignoring costs doesn’t make difficult choices disappear; it makes them less clear.”

Lomborg is often criticized for his focus on economic calculations—which, he has written, indicate that it would be less effective in the next twenty years to spend money on preventing climate change than on universal education, expanding trade opportunities, or cutting back on indoor pollution caused by poor stoves. In effect, Lomborg and others say that feel-good categories such as those in the S.D.G.s will only create the equivalent of a series of ice-bucket challenges, referring to the millions of showy dollars that were raised last year to address A.L.S.—a terrible disease, but one that afflicts relatively few people. For years, we have had tools such as the disability-adjusted life year (DALY), which was created in the early nineteen-nineties, by the World Bank, and has come to serve as the standard measure for assessing the burden of a disease. In the past, the impact of any illness—the common cold, cancer, and everything in between—was usually evaluated on the basis of how likely it was to kill you. But life without good health also carries enormous costs for individuals, families, and societies. The DALY combines years of potential life lost owing to premature death with years of productive life lost to disability.

That sort of principle, which associates benefits with costs, ought to be applied to massive investments in human development. Unfortunately, we need to choose which terrible blights we need to prevent and which we do not. People hate thinking that way (and they hate those who write about it). Nobody wants to put dollar values on a disease, a treatment, a life, an ocean, or the future of a country. But feel-good virtue alone rarely succeeds, and, if the Millennium Development Goals have demonstrated anything, it is that this planet and the people who live so tenuously on it will survive only if we spend our money on programs that work.

* This post has been revised to reflect that the U.N. relies on the World Bank's metric for "complete poverty."