Notes on the Index

The table of contents and the index perform two different functions. The table of contents provides a hierarchical structure (a tree), which is good for organizing and remembering, while the index provides an associative structure (a web or graph), which is good for associating and exploring. Well, sort of … this site is a representation of my mind, not yours, so the associative structure allows you to follow my associations, not make your own. There are other sites for that.

Although the backlinks aren't exactly an index, I originally hoped they might be usable as such. I had imagined that each essay would represent a single concept. All references to the concept would then refer to the corresponding essay, and the backlinks for the essay would be the index entries for the concept. I still think this is an interesting idea, but in practice it's not quite as satisfying as a normal index, for a number of reasons.

• The mapping between essays and concepts isn't as clean as I thought it would be. As one example, there are many familiar concepts that I'd like to include in an index, but that I don't actually have anything to say about.
• Not every reference to a concept is indicated with a link. Sometimes this is because the corresponding essay didn't exist when I was writing the essay, but more often it's because a link just isn't appropriate in the prose. Practically every other word refers to some concept or other; if every such word were a link, the important links wouldn't stand out as they should (low signal-to-noise ratio).
• To find the index entries for a concept, one first has to find the essay representing the concept. Although the table of contents and the index of essays by title are of some help, neither is as fast or as reliable as a normal index.

Speaking of indices, I'm very fond of the idea that the index entries for a particular concept should indicate the type of reference involved. For example, here's the system Knuth used for the index of The TeXbook.

Page numbers are underlined in the index when they represent the definition or the main source of information about whatever is being indexed. (Underlined entries are the most definitive, but not necessarily the easiest for a beginner to understand.) A page number is given in italics (e.g., ‘123’) when that page contains an instructive example of how the concept in question might be used. Sometimes both underlining and italics are appropriate.

The TeXbook also wins the prize for having genuine mnemonic titles for all ten appendices. The index, for example, is appendix I.