1) ''Like any emergent system, the city is a pattern in time. Dozens of generations come and go, conquerors rise and fall, the printing press appears, then the steam engine, then radio, television, the Web---and beneath all that turbulence, a pattern retains its shape: silk weavers clustered along Florence's Por Sanata Maria, the Venetian glassblowers on Murano, the Parisian traders gathered in Les Halles.''

2) ''There are manifest purposes to a city---reasons for being that its citizens are usually aware of: they come for the protection of the walled city, or the open trade of the marketplace. But cities have a latent purpose as well: to function as information storage and retrieval devices. Cities were creating user-friendly interfaces thousands of years before anyone even dreamed of digital computers. Cities bring minds together and put them into coherent slots. Cobblers gather near other cobblers, and button makers near other button makers. Ideas and goods flow readily within these clusters, leading to productive cross-pollination, ensuring that good ideas don't die out in rural isolation. The power unleashed by this data storage is evident in the earliest large-scale human settlements, located on the Sumerian coast and in the Indus Valley, which date back to 3500 B.C. By some accounts, grain cultivation, the plow, the potter's wheel, the sailboat, the draw loom, copper metallurgy, abstract mathematics, exact astronomical observation, the calendar---all of these inventions appeared within centuries of the original urban populations. It's possible, even likely, that more isolated groups or individuals had stumbled upon some of those technologies at an earlier date, but they didn't become part of the collective intelligence of civilization until there were cities to store and transmit them.''

3) ''Our brains first helped us navigate larger groups of fellow humans by allowing us to peer into the minds of other individuals and to recognize patterns in their behavior. The city allowed us to see patterns of group behavior by recording and displaying those patterns in the form of neighborhoods. Now the latest software scours the Web for patterns of online activity, using feedback and pattern-matching tools to find neighbors in an impossibly oversize population. At first glance, these three solutions---brains, cities, software---would seem to belong to completely different orders of experience. But as we have seen over the preceding pages, they are all instances of self-organization at work, local interactions leading to global order. They exist on a continuum of sorts. The materials change as you jump from the scale of a hundred humans to a million to 100 million. But the system remains the same.''