Niklas Luhmann was famous for his 'connected thinking note system (Zettelkasten).'
He had two note boxes. These two boxes were the core of his intellectual production system.
- Bibliographical slip-box
- Main slip-box
These two boxes were used to collect 'information from things he read' and 'his own thoughts,' respectively.
First, the bibliographical slip-box held bibliographic information from books and papers he read. Bibliographic information means the basic details that help you find the source again later -- book title, author, publisher, publication year, volume number, description, etc. Luhmann wrote this information on one side of a note card and jotted down impressive content or brief thoughts from the book or paper on the back. These accumulated notes were stored in the bibliographical slip-box.
In the next step, he would revisit his brief reading notes and consider how they connected to his own thoughts and writing material. When connections appeared, Luhmann would move to the main slip-box and write his own ideas and insights on a new sheet of paper. One sheet per idea, and only on one side! This way, he could easily flip through notes later without having to pull them out.
When new thoughts emerged, Luhmann would write additional notes to expand his thinking, always referencing existing notes while creating new ones. The key point is that he didn't simply copy quotations -- he translated the context of what he read into the context of his own thinking.
Luhmann's notes had an interesting characteristic. He didn't organize notes by topic. Instead, he assigned unique numbers, which served not as indicators of content but as permanent identifiers for distinction. For example, after note 22, if a new related note appeared, it became 23 or 22a. Sometimes he mixed in symbols like / and , to extend branches of thought. For instance, 21/3d7a6 might be followed by 21/3d7a7.
Also, every time he added a new note, he would find related notes and add link numbers. By connecting notes to each other this way, Luhmann could naturally move from one thought to another anywhere in his box. Rather than a top-down approach of defining topic structure first, it was a bottom-up approach of starting from individual thoughts and gradually building a system.
And the last important element was the index. The index served as an entry point into various thoughts and topics. Luhmann created 'entry notes' for major topics and gathered links to related notes there. So just by looking at the index, you could see a map of all the topics he was exploring.
Luhmann's Note-Taking Process#
Step 1. Record Bibliographic Information
When reading a book or paper, Luhmann wrote bibliographic information (book title, author, publisher, publication year, volume, etc.) on one side of a note card.
This information was stored in the bibliographical slip-box for future reference.
Step 2. Summarize What You Read
On the back of the note, he briefly wrote down impressive sentences, ideas, and observations from the book or paper.
But he didn't stop at simple quotes or summaries -- he reinterpreted them in his own words.
Step 3. Expand into Your Own Thinking
By reviewing his bibliographic notes, he identified parts that could be developed into his own ideas or commentary.
Those thoughts were transferred to new paper, and these became the notes that went into the main slip-box.
Step 4. One Idea Per Sheet
Luhmann wrote each idea on a separate sheet of paper.
He only used one side so he could easily flip through and review them later. When needed, he wrote new related notes to expand his thinking.
Step 5. Connect to Existing Notes
Every time he recorded a new note, Luhmann compared it with existing notes.
If there was a relevant existing note, he would add the new note next to it and assign a unique number.
For example, the note after number 22 would become 23 or 22a.
Step 6. Expand Thinking Through a Numbering System
Luhmann didn't attach meaning to numbers -- he used them purely for identification.
When needed, he mixed symbols like / and , with numbers and letters to freely extend branches of thought.
Example: 21/3d7a6 followed by 21/3d7a7.
Step 7. Add Links Between Notes
Every time he created a new note, Luhmann added link numbers to related notes.
This allowed him to easily find connected thoughts from anywhere in the box.
Looking at the connection structure of notes, topic flows naturally emerged.
Step 8. Bottom-Up Structuring of Thought
Unlike other researchers who predefined topic categories, Luhmann started from small thoughts and kept connecting them,
ultimately thinking in a bottom-up manner where topics naturally became organized.
Step 9. Create Entry Points with an Index
Finally, Luhmann created an 'index' to provide entry points into important topics and thoughts.
A single index note contained links to multiple related notes, serving as a kind of 'thinking map.'
Thanks to this approach, Luhmann never got lost among his vast sea of ideas.
His note box wasn't just a collection of notes -- it was a 'system where thoughts connect.'
Tools we use today like Obsidian, Notion, and Roam Research can actually be traced back to Luhmann's ideas.
Ultimately, Luhmann's note system wasn't just an organizational method -- it was a thinking tool for expanding ideas and connecting knowledge. Through his notes, he didn't just accumulate 'knowledge' -- he built a 'network of thought.'
Those who are blessed with the most talent don't necessarily outperform everyone else. It's the people with follow-through who excel.
— Mary Kay Ash
