The problem with trees Many systems are organized hierarchically. The CERNDOC documentation system is an example, as is the Unix file system, and the VMS/HELP system. A tree has the practical advantage of giving every node a unique name. However, it does not allow the system to model the real world. For example, in a hierarchical HELP system such as VMS/HELP, one often gets to a lead on a tree such as: HELP COMPILER SOURCE_FORMAT PRAGMAS DEFAULTS only to find a reference to another leaf: Please see HELP COMPILER COMMAND OPTIONS DEFAULTS PRAGMAS and it is necessary to leave the system and re-enter it. What was needed was a link from one node to another, because in this case the information was not naturally organized into a tree. Tim Berners-Lee, Seeing With Fresh Eyes A City Is Not a Tree hierarchywww
Cool URIs don't change An Essay by Tim Berners-Lee www.w3.org What makes a cool URI? A cool URI is one which does not change. What sorts of URI change? URIs don't change: people change them. The User Interface of URLs www
Guided by image In our minds, the drawings we had originally made for the columns and capitals were no more than first approximations of the final shapes. We assumed that we would work out the real shapes during construction, and left the inaccurate approximations on our drawings, just for the sake of the building permit. Fujita, used to working with architects in System B, assumed that whatever was on our drawings must be what we wanted, and must be implemented as drawn. Anybody who was making those column capitals, if they had seen this "double" capital, and had been free to make something harmonious, would have done it differently. But Fujita's people, in System B, did not know how to be guided by reality. They were guided by "image". So Fujita, in this situation, was not free to respond in a natural way to what they saw. They were trapped by the image-making process they were used to. But because of this, they doomed their own carpenters to a pretentious kind of slavery, producing whatever silly images they were told to do, without being able to ask themselves whether they were beautiful, and unable to use their own sense of reality to make them better. Christopher Alexander, The Battle for the Life and Beauty of the Earth Maps and observation design