Wiki is definitely not a tree. It is helpful to organize wiki, but it should not be categorized simply or hastily.
The Addition of a Category Tag does not a method make. It is simply a notation which takes advantage of an existing artifact which allows the lookup of BackLinks. This does not make a tree, it makes a list. If you want a tree, you will have to use external artifacts. The simple addition of a Category to a page is to provide a NavigationalConvenience.
You do not automatically take advantage of this convenience by viewing a page, you do so by first clicking the Category Tag, and secondly clicking on the title of the Category page. If you do not want to take advantage of Categories, simply ignore them. There are some who do like the convenience and find it to be a useful way to navigate the wiki. -- DonaldNoyes 20080107
PleasePleaseDoCategorizeEveryPageOnWiki and PleasePleaseDontCategorizeEveryPageOnWiki are opposing arguments that can both draw strength from WikiIsNotaTree.
See also: SystemsThinking
Inspired by: "A city is not a tree" www.rudi.net/bookshelf/classics/city/alexander/alexander1.shtml ( BrokenLink EditHint fix if possible, or else delete if impossible - is there a WikiPage discussing this article ?)
Note: The term tree is referring to the mathematical tree, which contains no cycles.
It maybe would not hurt to attempt a tree-based reference. It may be a UsefulLie for some people.
See WikiCategories LimitsOfHierarchies