A city is not a tree, is one of Christopher Alexander's most cited papers on urban design. I've been wanting to understand this idea better; here's my Twitter summary of his paper.

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Summary

Our brain's first priority is to help us deal with complexity by neatly grouping and categorising things into existing "columns and rows", resulting in tree-like thinking.

The problem with tree structures is that they lack structural complexity and thinking in trees, limit our ability to create rich and living cities.

The solution to this problem lies with how natural cities have evolved spontaneously over time into much more complex, semilattice structures.

Semilattice structures are characterised by rich overlap created by the fact that all elements within it interact with each other.

One of my favourite examples illustrating a semilattice structure is a story of how a traffic light, newsrack, money and people work together.

In Berkeley, at the corner of and Euclid, there is a drug store, and outside the drug store a traffic light. In the entrance to the drug store, there is a newsrack where the day's papers are displayed. When the light is red, people who are waiting to cross the street stand idly by the light; and since they have nothing to do, they look at the papers displayed on the newsrack which they can see from where they stand. Some of them just read the headlines, others actually buy a paper while they wait. This effect makes the newsrack and the traffic light interdependent; the newsrack, the newspapers on it, the money going from people's pockets to the dime slot, the people who stop at the light and read papers, traffic light, the electric impulses which make the lights change, the sidewalk which the stand on form a system - they all work together.

To have this same richness in cities, we can't simply recreate the overlap we find in historic cities; this results in chaos and what we often experience in modern, artificial cities.

To create cities that are like great paintings & beautiful symphonies, we need to study the hidden, more abstract parts of a city, things like:

• More complex interactions,
• Economic & political influences, and
• Social dynamics