"What is a city but people?", Shakespeare asked in the 16th century, when barely two percent of the world's then 500 million people lived in cities.

But to ask that question in this, the first century in which the majority of the world's people will live in urban areas, is to do so with the under- standing that the city is now more than three billion people — an increase of over 30,000 percent in just four centuries — the net effect of which is natural and cultural transformation on a scale unprecedented in human history.

Therefore...