A City Is Not A Tree ~ Alexander on Semilattices

I was an EECS undergrad at MIT, but I took many optional electives in Architecture, Sociology, Civil Engineering, Business, and beyond. This was fantastically important in exposing me to a rich intellectual smörgåsbord I would otherwise never have tasted. And among the very most important such nuggets was reading Christopher Alexander's A City Is Not A Tree where he decries the overly simplistic rules dominating (and debilitating) urban planning and design...

"Why is it that so many designers have conceived cities as trees when the natural structure is in every case a semilattice? Have they done so deliberately, in the belief that a tree structure will serve the people of the city better? Or have they done it because they cannot help it, because they are trapped by a mental habit, perhaps even trapped by the way the mind works -- because they cannot encompass the complexity of a semilattice in any convenient mental form, because the mind has an overwhelming predisposition to see trees wherever it looks and cannot escape the tree conception? I shall try to convince you that it is for this second reason that trees are being proposed and built as cities -- that is, because designers, limited as they must be by the capacity of the mind to form intuitively accessible structures, cannot achieve the complexity of the semilattice in a single mental act. [...] For the human mind, the tree is the easiest vehicle for complex thoughts. But the city is not, cannot and must not be a tree. The city is a receptacle for life. If the receptacle severs the overlap of the strands of life within it, because it is a tree, it will be like a bowl full of razor blades on edge, ready to cut up whatever is entrusted to it. In such a receptacle life will be cut to pieces. If we make cities which are trees, they will cut our life within to pieces."

Wow!