One of the major affordances of using a zettelkasten or card index for writing is that it forces the writer to break things down into their constituent parts, thereby making the entire process of writing far easier and less complex. One can separately focus their attention on the smaller steps of collecting, selecting, and arranging the material before beginning to actually write.

Julius Reizen in reviewing over Umberto Eco's index card system in How to Write a Thesis, defines his own "insurance card" as one which contains "hard-to-classify thoughts, questions, and areas for further inquiry". These he would keep together so that they don't otherwise get lost in the variety of other locations one might keep them

These might be akin to Ahrens' "fleeting notes" but are ones which may not easily or even immediately be converted in to "permanent notes" for one's zettelkasten. However, given their mission critical importance, they may be some of the most important cards in one's repository.

link this to - idea of centralizing one's note taking practice to a single location

Is this idea in Eco's book and Reizen is the one that gives it a name since some of the other categories have names? (examples: bibliographic index cards, reading index cards (aka literature notes), cards for themes, author index cards, quote index cards, idea index cards, connection cards). Were these "officially" named and categorized by Eco?

May be worthwhile to create a grid of these naming systems and uses amongst some of the broader note taking methods. Where are they similar, where do they differ?

Multi-search tools that have full access to multiple trusted data stores (ostensibly personal ones across notebooks, hard drives, social media services, etc.) could potentially solve the problem of needing to remember where you noted something.

Currently, in the social media space especially, this is not a realized service.

p xxvi

Hutchins closes his preface to his grand project with Mortimer J. Adler by giving pride of place to Adler's Syntopicon.

Adler's Syntopicon isn't just an index compiled into two books which were volumes 2 and 3 of The Great Books of the Western World, it's physically a topically indexed card index of data (a grand zettelkasten surveying Western culture if you will). It's value to readers and users is immeasurable and it stands as a fascinating example of what a well-constructed card index might allow one to do even when they don't have their own yet.

Adler spoke of practicing syntopical reading, but anyone who compiles their own card index (in either analog or digital form) will realize the ultimate value in creating their own syntopical writing or what Robert Hutchins calls participating in "The Great Conversation" across twenty-five centuries of documented human communication.

See also: https://hypothes.is/a/WF4THtUNEe2dZTdlQCbmXw

The way Hutchins presents the idea of "Adler's achievement" here seems to indicate that Hutchins didn't have a direct hand in compiling or working on it directly.

A partial answer to note taking why.

It's quite likely that one will take a large number of notes with no immediate goal or plan for use and this is completely acceptable. Often these notes go towards the more immediate goal of forming one's own understanding and setting of a broader tableau for material one will write in the future.