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This is a note-taking system designed by Niklas Luhmann, a sociology professor who used it to write an astonishing 58 books and hundreds of articles over 30 years.
The concept behind it is to use index cards or slips of paper to capture individual ideas, one for each slip of paper, and place them into your slip-box (Luhmann’s slip box appear to be the drawers of a library catalogue). These individual notes are then linked together through references and links of other cards.
The beauty of the system is that nothing ever falls into the void—that collection of notes and ideas that you write down and never reference again. Instead, by linking them together, they generate clusters representing your interests and passions, which you can then turn into research papers or essays.