This is the famous Zettelkasten of the systems theorist Niklas Luhmann in a video from 1989.
Now the University of Bielefeld has put it online.
It is in German but it would be a pity if it missed a wider audience, because it is not only an interesting sociology resource, but it has an incredible wealth of cross references.
So I updated my think tool to try some visualizations of its links. The most peculiar thing is the hierarchical numbering scheme of the zettels. I tried to visualize these as a tree, at least the first thousand zettels. But navigating on such a large map, you can get seasick.
The editors in Bielefeld have assembled some “threads”. They do not closely correspond to the numbering scheme. I tried to visualize the relationship in a long map showing the threads alternately as red and black. But you can still get seasick. These threads show a very linear view of the network.
Therefore I created a third view option showing all the cross references. Here I think you will not lose your orientation, even if some long jumps are occasionally needed. Normally you can navigate within the neighborhood.
If necessary you can also start the demo directly in the brwoser, but this is a very limited version.