AP Photo

Throughout much of history, cities have served as incubators for upward mobility. A great city, wrote René Descartes in the 17th century, was “an inventory of the possible,” a place where people could lift their families out of poverty and create new futures. In his time, Amsterdam was that city, not just for ambitious Dutch peasants and artisans but for people from all over Europe. Today, many of the world’s largest cities, in both the developed and the developing world, are failing to serve this aspirational function.