CONSIDER THE PROBLEMS which beset the frenetic mega-city of your choice — not just New York or Los Angeles, but places like Mumbai, Rio de Janeiro, Cairo. Each has its slums, its political morass, its dangerous, dispersed particulates in the air. It's tempting to imagine the best solution to the chaos is an unspooling: level the fire-trap tenements, empty the choked highways, shut the dangerously sub-par schools, run those few, prosperous global traders out of their well-appointed penthouses at the point of a pitchfork. End the misery, once and for all. You could feel that way about any of the above cities, or even about a hundred cities in China you've never heard of, some of which are expanding by an entire Pittsburgh every year. Listen to the bottom-rung stories of global commerce: suicidal workers in China's booming factory cities, life in the no-go zones of Rio's favelas. That's enough to make you want to send us all back to the land.

And yet, for all the smog and political corruption and inequality, our clustering in cities has a logic. It is neither an accident nor a disaster that humanity is now, for the first time in its history, a predominantly urban species. The rationale for city life is not simply economic efficiency, and it is not simply the giddy cosmopolitanism of art museums and restaurants and architecture. What cities offer is contact: "super-linear" contact, as the Santa Fe Institute calls it, exponential connections among people. This contact breeds innovation, the production of more and better knowledge. If an increasingly urban way of life is our collective future, now would be a good time to refine the conversation, to pay better attention to what cities do well and get a sharper handle on the problems for which we can fault them.

For help in this task, we have Edward Glaeser's thrillingly optimistic account of our current urban moment and its future prospects. Rather than environmental despoliation, for example, Glaeser encourages us to see efficiencies that vastly outweigh the environmental costs. Glaeser contends that people aren't crushed by cities, but ennobled by them; that cities attract poverty much more than they create it; and that, for the world's rural poor, urban life offers a level of economic and social mobility that far outstrips the life offered by subsistence agriculture. Resurrect Jane Jacobs as a Harvard economist with a sharp eye for government policy wrinkles and unintended consequences, and you get Glaeser's The Triumph of the City.

Glaeser's vision of "triumph" really bifurcates into two related themes. The first is a historical brief about the fantastic capacity of cities —"our species' greatest invention" — to make us think, live and act in new and productive ways. Think of the ideas cities have produced: democracy, algebra, Renaissance art, advanced surgery. Modern day Baghdad may be at its low ebb, but in antiquity it was a famously successful metropolis. The Abbasid caliphs brought a compendium of scholars to the city's "House of Wisdom," where the world's extant knowledge was translated into Arabic, and whence knowledge of the Old Testament, Indian mathematics, Hippocrates, Plato, and Aristotle passed into the Arab world. It was Chinese prisoners of war that brought the secret of paper-making to Baghdad; it seems the city, as an innovation engine, isn't terribly picky about how one arrives there. If you can attract or conscript enough people into the same dense space, then one person's radical idea will quick...