Zettelkasten

The Zettelkasten (“slip box”) is the note-taking method of the sociologist Niklas Luhmann, who used tens of thousands of index cards to write dozens of books. Sönke Ahrens’ How to Take Smart Notes brought it to everyone else. I started my own in 2020, at university, and these principles have survived every tool change since.

The method

1. Literature notes. While reading, take brief notes, in your own words, with no copy-pasting. Quote only to reference the source.
2. Reference notes. Record where each idea came from.
3. Permanent notes. Later, turn literature notes into full-sentence notes, one idea each, and connect them to what you already know. This is the step that counts.
4. Review and repeat.

Learning is not so much about saving information like on a hard disk, but about building connections and bridges between information. — Sönke Ahrens (paraphrased)

Principles that stuck

• One slip box for life. Not one per course or project. A single, growing wiki of what you know.
• Atomic notes. One idea per note, so it can be linked from many places.
• Links over folders, tags over categories. An idea about feedback loops belongs to robotics, management and handstands at once.
• Your own words. You have to interpret a source to write about it, and interpreting is learning. See Comprehension Requires Your Own Words.
• Avoid the collector’s fallacy. Bookmarking, highlighting and buying books feel like learning. They aren’t. Work with the material: write, connect, build.
• Buffer the influx. New material goes into temporary notes first, to be sorted and rewritten later. Not everything deserves a permanent note.

Layers of evidence

A framing I like for permanent notes: describe what you saw, interpret why you saw it, then synthesise something useful from it. Most notes stall at the first layer.

Writing grows out of it

Start a piece of writing from an outline, then flesh it out by pulling in existing notes. With a good slip box you never face a blank page, because the thinking has already been done in small pieces. This garden is exactly that: notes written over years, rewritten and published.

In practice

My early slip box was too eager. It had many tiny notes that were really just quotes. What fixed it was the review loop in LOGREDIS: only promote what proved useful, and rewrite it properly when you do. The broader why is in The Second Brain, and how to make permanent notes actually stick in memory is in Spaced Repetition.