Cities are like compost heaps

Cities are like compost heaps — just layers and layers of stuff. In cities, the past and the present and the future can all be totally adjacent.

William Gibson

And also:

Cities look to me to be our most characteristic technology. We didn’t really get interesting as a species until we became able to do cities—that’s when it all got really diverse, because you can’t do cities without a substrate of other technologies. There’s a mathematics to it — a city can’t get over a certain size unless you can grow, gather, and store a certain amount of food in the vicinity. Then you can’t get any bigger unless you understand how to do sewage. If you don’t have efficient sewage technology the city gets to a certain size and everybody gets cholera.

From reading Leonardo da Vinci, I gather this is exactly what happened in medieval cities. The city got to a certain size, and everyone got cholera. People packed behind city walls for safety — disease and violence were accepted as background noise.

da Vinci's sketchbooks contain elaborate plans for Renaissance utopias with systematic plumbing, water, and garbage disposal. But utopias don't get built, or when they do, we discover they're missing important properties of evolved systems (human scale, evolution).

Cities accrue — people, technology, opportunity. It's hard to beat that network effect, even when factoring in the mess. Cities outlive civilizations.

There is no logic that can be superimposed on the city. People make it, and it is to them, not buildings, that we must fit our plans.
— Jane Jacobs