### Coherent cities

What holds a city together? Rose noted that the earliest cities were built around a temple and the spirituality it embodied. As the early communities became larger and more diverse and complex, their economic activity intensified. To be effective in trade they had to specialize, monetizing their regional opportunities. One city became known for shipping, another for serving caravans. One as a source of metal, another as a source of grain.

To cope with their growing complexity the cities had to develop varying control systems for everything—irrigation, food storage, accounting, building codes. The Code of Hammurabi was written in 1754 BCE explicitly “to further the well-being of mankind.” (One of its building-code provisions declared, “If your building falls down and kills somebody, we kill you.”)

Modern cities need to create their own “circular economy,” Rose stressed, not just of services and goods, but of greener waste treatment, of water recycling, of food creation (such as“vertical gardens”,) and especially of what he called "communities of opportunity”—where low-income groups such as immigrants get a chance to create prosperity for themselves and the city.

In his own many real-estate projects, Rose focusses on increasing urban density with low-income housing in combination with improved mass transit, local parks, better schools, and the greenest of building standards. But for such innovations to be copied, he pointed out, they have to be profitable.

Cities are systems, Rose concluded: “When a system is optimized, then all of its components do well. Cities that focus on the optimization of the whole for everybody are the ones that thrive the best.”