In previous comments, I have argued that what makes cities different than building projects was the fact that they have to deal with change and uncertainty, and that subdivision-planned developments are economically inferior to random growth. These arguments rely on the fundamental quality of cities as systems, a property that places them in the same class as biological systems while separating them from mechanical systems. This quality is being scale-free. That is to say, a city can work no matter what size it takes.

The ability of a system to function at multiple scales is behind the growth process of all multicellular lifeforms. It all starts as a single embryo, multiplying into thousands and millions and billions of cells. These cells work together to emerge the form of a sapling, which immediately begins to function autonomously as it grows into a full-sized tree. The processes in the DNA of a tree are able to function at whatever scale the tree grows. They can work even if half the branches are cut off, for example to make one of those distinctively-French square trees.

It should be obvious that this is a radically different quality than those possessed by mechanical system. We cannot imagine a car growing with us over the years. We cannot imagine a car working if one of the wheels is taken away. In a mechanical system, action is linear. If one system or sub-system fails, the whole structure fails. In a scale-free system, no single sub-system is that critical, although they each have a marginal impact on the total efficiency of the structure. So a tree might not die from being cut square, but it will not function as efficiently.

The idea of creating something whose size is not going to be known is alien to engineering and architectural practices. But this is not to say that it has never been done before. The Internet is without a doubt a great achievement of scale-free system design. Its foundations, Arpanet, was intended by the military men to be a communications system that could function through a nuclear war, which implied a catastrophic loss of infrastructure in random places. The cables and links that you are using relate directly back to this original system, and they have grown to such a scale that no one really knows how big it is. If it works, don't fix it. But how does it work?

The idea of a network that could continue to function despite bombardment was actually demonstrated in World War II, when large-scale strategic bombings of cities devastated Germany and Japan. Quite surprising was the fact that, instead of resulting in a massive exodus of urban populations to the countryside, leaving ghost towns behind, bombed cities continued to function, supporting the lives of their residents and industrial war effort, although with greater hardship. Despite catastrophic reduction in scale, cities adapted and continued to work. The modernist plans for cities of 1,000,000 people of the time were set up to fail. By designing in advance the final form of a city as if it were a building project, just bigger, modernists failed to understand the fundamental benefits of cities. Even those plans that were realized, like Brasilia, face intense pressure to change their scale and grow new relationships, as witnessed by Mr. Bill Hillier.

The systems that allow the internet to work are founded upon relational rules. It is by defining protocols for how different networks relate to one another that all of them come together to form the Internet, without any of them being really aware of the scope of the entire system. The form the system takes is fractal. (A fractal is a relational rule applied repeatedly.)

The most simple form a city can take is that of a village on a road. But what is the difference between one village in the countryside, and 100,000 villages in a metropolis? It is the spaces that tie them together at a larger scale. From an "urban village" where your house is you enter an avenue, which has shops and activities and businesses along with faster movement. The avenue relates the villages together, and the grand avenues relate the avenues together. The expressways link the grand avenues, all the way up to the airports who link the cities together. Building these relationships is the basic day-to-day work of city corporations.

Relational rules also appear in the essential tool of urban planning, the building code. Building codes ideally allow the maximum flexibility in local problem-solving, the design of a building, while integrating the building smartly into the urban fabric of the city as a whole. A good building code is itself scale-free. It defines how anything from a bungalow to a soaring skyscraper is to be shaped in order to be compatible with the whole.

If done right, a city plan will work beautifully whether it is growing or shrinking, whether it has ten inhabitants or ten million. The job of designing cities is not so much about determining form, but about defining the processes that will generate their form.