- Zettelkasten Method is a method of note-taking and organization. It is German for "slip box".
- Popularized by Niklas Luhmann
- Great for building a second brain
- Can help find connections between notes
- Useful for thinking more broadly by not confining your notes to a subject
So basically, the Zettelkasten Method relies on interconnectivity of information, and atomicity of concepts. You make notes that work independently. If you feel a connection, you note it down.
Luhmann's Zettelkasten - The Method in His Words
Luhmann used a physical box of cards. Made 90,000+ handwritten notes. Some of Luhmann's comments on the method (emboldening by me) -
“Underlying the filing technique is the experience that without writing, there is no thinking.”
“I always read with an eye towards possible connections in the slip-box.” 
“I, of course, do not think everything by myself. It happens mainly within the slip-box.”
(The zettelkasten is a) “combination of disorder and order, of clustering and unpredictable combinations emerging from ad hoc selection.”
"Probably the best method is to take notes – not excerpts, but condensed reformulated accounts of a text. Rewriting what was already written almost automatically trains one to shift the attention towards frames, patterns and categories in the observations, or the conditions/assumptions, which enable certain, but not other descriptions.”*
— Niklas Luhmann
Principles of Zettelkasten
- Note taking - Take notes. Not excerpts, but how you understand it. In your own words.
- Atomicity - He realised that one idea, one note was only as valuable as its context, which was not necessarily the context it was taken from. So, one idea per note.
- Autonomy - Notes should make sense by themselves too.
- Connect Notes - Try to link all notes. "A note that is not connected to the network will be lost, will be forgotten by the Zettelkasten"
- Number your notes - use any numbering system and stick to it. It's just a way to identify the cards for linking. The numbers do not mean anything. The point of the method is not having to worry where a note goes.
The core of the technique lies in a few simple principles:
- Write liberally about what you think and read. Write in detail so you can quickly re-internalize ideas after you forget them.
- Wherever you can, separate notes into small and distinct ideas.
- When you notice connections between ideas, encode them by writing references from one note to another.
- When you want to think or write, read your notes and follow the references between them.
- Update as you go. Find something new to add? Add it. Don't agree with something you had written once? Don't trash the old note! Make a connection to the new one and explain why you changed your mind.
- Niklas Luhmann was a sociologist and a prolific writer having published as many as 70 books and nearly 400 scholarly articles. His main work was in Systems Theory. Luhmann himself described his theory as "labyrinthine" or "non-linear" and claimed he was deliberately keeping his prose enigmatic to prevent it from being understood "too quickly", which would only produce simplistic misunderstandings!