Known for his detailed analyses of the architectural design process, here Alexander attempts to juxtapose the structure of those cities that have grown organically with those that have been intentionally designed on the Architect's drawing board, and in particular how this can lead to urban zones that lack any sense of 'the patina of life'. He argues that this uniformity is a product of our tendency to try and give order to complex sets of information - in order to make them more palatable - even in those circumstances where their complexity may be the very foundation of their value.
To achieve this he begins by classifying the two extremes of a continuum of _structured sets of information_ (information that can be grouped together for some reason): from _strict hierarchical trees_ (where each _unit of information_ can only be linked in a hierarchical nested fashion), through to _open semi-lattices_ (where each unit of information can be linked as overlapping collections). In particular, he notes how the use of nested tree structures vastly reduces the potential interconnections that can be made between the members of an information set, and is therefore a natural tool for a human aiming to reduce its complexity.
Whilst the essay is focused in the role of the architect in designing urban spaces, Alexander's thesis can also be applied to the management of information in general. In particular, the two extremes of his continuum provide a useful analogy for the design of information management tools and the functionality that they offer the user to structure the information that they contain, such as can be achieved through the strict hierarchical tree of an outline processor, or the semi-lattice approach taken by open hyper-linked systems, such as the World Wide Web.
As argued by Alexander, the human desire to give information a nested tree-like structure is a very natural one - being a product of the ways in which our brains try to reduce complexity - and speaks to the reasons why the popularity of outliners has persisted as the technological landscape continues to shift. However, as Alexander notes, there is a cost to enforcing this tree-like structure, and so system designers must consider how the user might be supported in breaking out of this structure when a looser graph of interconnection is required.
How then might the pattern for Outline Processors be expanded to take account of this semi-lattice of interrelations found in complex information sets? Approaches include the introduction of tagging or labeling where, in addition to the strict parent-child relationship, each node can be related to any other through an open categorisation system which allows sets of nodes to be displayed together, no matter where they were originally located in the originating tree. Another alternative is to allow nodes to be directly related to one-another, or even to provide functionality to allow each node to be located under multiple parents at the same time.