In September next year, the United Nations plans to choose a list of development goals for the world to meet by the year 2030. What aspirations should it set for this global campaign to improve the lot of the poor, and how should it choose them?

In answering that question, U.N. Secretary-General Ban Ki-moon and his advisers are confronted with a task that they often avoid: setting priorities. It is no good saying that we would like peace and prosperity to reach every corner of the world. And it is no good listing hundreds...