I've read an awful lot of novels and not a single one has had an index.
I was told similes have to have the word 'like' or 'as' in them. Which is just not true but has become such a common hueristic that even English teachers think its the definition of a simile. sigh
I totally did that on purposes though cause I wanted to see if she would dock me a point for that. I was such a little prick.
Footnotes. I hadn't thought about footnotes. The winner here has to be Jonathon Strange and Mr Norrell, some of whose footnotes are micro stories thrown in as a bonus for attentive readers.
Although Jonathon Strange and Mr Norrell doesn't have an index.
It's used in domains where access is based on physical location (as with libraries), though there are numerous alternatives to subject-based grouping used (especially with closed stacks).
If you ever want to see librarians in a heated discussion, tell them of your plans to develop a consistent hierarchical topical classification scheme.
That said: there are useful classifications. None are perfect, serve all needs, please all parties, or are fully consistent. But most are better than a haphazard mess.
You'll find that:
1. It's harder than it looks. 1. The hierarchy is stubbornly inconsistent.
Take the example of MP3s, or more generally, audio files vs. text vs. images vs. multimedia.
Do you classify all audio indepednetly of any text, regardless of whether it's, say, a history of WWII in the Czechoslovakia?
Is a historical study of military technology used in Czechoslovaki during WWII filed under history (WWII), geography (Czechoslovakia), technology (military), or military science (technology)?
What happens when countries change or borders shift (Holy Roman Empire -> Austria-Hungary -> Czechoslovakia -> Czechia)?
Or when a region is disputed (West Bank, Taiwan, Kashmir, Oregon Teritory, Malvinas, ...)? Is Ireland part of the British Isles?
Is it "mythology", "fiction", "religion", "theology", "philosophy", "cult", or "mass delusion"?
Even if it's a single person's classification, conflicts rapidly emerge. Increase the number of catologers, or add multiple readers/users, and you'll start getting complaints and exceptionally strong disagreements.
I'm not opposed to the practice. I've attempted it myself numerous times. I'm working on several approaches presently.
How about tag-based systems as an alternative to a hierarchical one?
The tags can be presented as a hierarchy, if you like. Or better, as several different hierarchies, or perhaps alternatively, as a web or space with multiple entry points. Depending on how you slice it (and where that slice begins and ends) you'll end up with different organisations, though each will have at least some internal consistency.
The hierarchical ontology is a DAG. It's got a root, it propagates from the root to leaf nodes, and there is no recursion.
A fully tag-based system is neither directed nor acyclic. My view is that the better ones appear as DAGs withiin a localised area, but that constraint falls at a broader level.
The main problem with tag-based systems is that they tend strongly to be "folksonomies", where terms are not controlled and there can be multiple variants of spelling, naming, etc., often reflecting both personal, geographic, and temporal preferences / ideosyncracies.
That's where a controlled vocabulary or defined set of relationships / associations (perhaps probabalistic) helps. E.g., "Yerba Buena" -> "San Francisco", "Frisco" -> "San Francisco".
The Library of Congress Classification and Subject Headings make for an interesting study. They're quite extensive and have evolved over more than a century. They have their own inconsistencies, ideosyncracies, and controversies. Part of their strength is the surrounding infrastructure establsihed to support and address these. There are of course other classifications.
> It's got a root, it propagates from the root to leaf nodes, and there is no recursion.
A network of instances and tags satisfies the same properties. Do you have any counterexample? If not, I have to dismiss a notion that "tag-based systems" stand in opposition to "hierarchical systems" since every network of objects and tags is a DAG.