Niklas Luhmann’s Zettelkasten is a personal knowledge management system based on interlinked note cards, where each note (or Zettel) captures a single idea or concept. Notes are sequentially numbered, and links between them form a web of connected thoughts, allowing complex ideas to emerge through associative connections. This system is self-referential, enabling non-linear exploration of topics, making it highly effective for developing insights over time. Luhmann credited it for his prolific output, including over 70 books and 400 scholarly articles. One of the precursors of Digital Gardening.