More from A City is not a Tree. Christopher Alexander asks, “Now, why is it that so many designers have conceived cities as trees when the natural structure is in every case a semilattice?” He answers:

[B]ecause 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. [–––]

This is the problem we face as designers. While we are not, perhaps, necessarily occupied with the problem of total visualization in a single mental act, the principle is still the same. The tree is accessible mentally and easy to deal with. The semilattice is hard to keep before the mind’s eye and therefore hard to deal with.

I wonder what the implications of this are for software development. Does up-front planning more often result in tree-like architectures? Do evolutionary methods result in semilattices? Are semilattices even preferrable? Does it matter? Alexander continues this train of thought:

It is known today that grouping and categorization are among the most primitive psychological processes. [–––] Study of the origin of these processes suggests that they stem essentially from the organism’s need to reduce the complexity of its environment by establishing barriers between the different events that it encounters.

[T]he mind’s first function is to reduce the ambiguity and overlap in a confusing situation and because, to this end, it is endowed with a basic intolerance for ambiguity.

This is why, he argues, we conceive of structures like the city as trees. The problem is that designers, planners, and such, can’t continue to use trees – Alexander writes that it means “we are trading the humanity and richness of the living city for a conceptual simplicity which benefits only [them].”