The final shape of any one particular oak tree is unpredictable ... And a town which is whole, like an oak tree, must be unpredictable also.
The fine details cannot be known ahead of time. We may know, from the pattern language which is shared, what kind of town it will be. But it is impossible to predict its detailed plan: and it is not possible to make it grow according to some plan. It must be unpredictable, so that the individual acts of building can be free to fit themselves to all the local forces which they meet.
The people of a town may know that there is going to be a main pedestrian street, because there is a pattern which tells them so. But, they cannot know just where this main pedestrian street will be, until it is already there. The street will be built up from smaller acts, wherever the opportunity arises. When it is finally made, its form is partly given by the history of happy accidents which let the people build it along with their more private acts. There is no way of knowing, ahead of time, just where these accidents will fall.
This process, exactly like the emergence of any other form of life, alone produces a living order.christopher alexander, the timeless way of building