Niklas Luhmann, the son of a brewer in Germany, graduated from law school in the late 1940s and became a public administrator. Not much for socializing, he read after his typical 9-5 shift on a wide array of topics like sociology, philosophy, politics and social systems theory. Like many people, Luhmann wrote notes in the margins, placed slips of paper with handwritten notes between pages and even organized notes by topics of interest in folders. Unlike many people, Luhmann noticed the meager fruits of his labor. The notes he collected did not amount to much.
With a simple shift in thinking, Luhmann created his own method of note-taking called Zettelkasten, which is German for "note-box" or "slip-box." In a nutshell, he read things of interest, took notes in his own words on slips of paper, numbered them and stored them in his slip-box — small beech wood cabinets with drawers. The numbering system was not just a reference for individual notes. It was a simple mechanism for branching and linking related ideas such that the slip-box became more than the sum of its individual notes. It became his playground of thought.
I, of course, do not think everything by myself. It happens mainly within the slip-box.
— Niklas Luhmann
Luhmann's slip-box, with a final tally of approximately 90,000 notes, certainly does not look like much. It consists of six small cabinets with four drawers each, no larger than a dresser overall. And yet, that slip-box fueled an astonishing 58 books and hundreds of scholarly articles within a 30 year period. An additional 12 books were published posthumously in his name from unfinished manuscripts. As Sönke Ahrens quips in How to Take Smart Notes, "[t]here are more than a few colleagues I know who would give a lot to be as productive in their whole lifetime as Luhmann was after his death."
Luhmann interacted with his slip-box as if it were a sentient being full of its own arguments and insights, a secondary memory that became a communication partner and alter ego.
As a result of extensive work with this technique a kind of secondary memory will arise, an alter ego with who we can constantly communicate. It proves to be similar to our own memory in that it does not have a thoroughly constructed order of its entirety, not hierarchy, and most certainly no linear structure like a book. Just because of this, it gets its own life, independent of its author.
— Niklas Luhmann (Communicating with Slip Boxes)
This alter ego did not form overnight. It took a few years for Luhmann's slip-box to reach a critical mass of maturity, namely size and complexity.
Naturally, independence presupposes a minimal measure of intrinsic complexity. The slip box needs a number of years in order to reach critical mass. Until then, it functions as a mere container from which we can retrieve what we put in. This changes with its growth in size and complexity.
— Niklas Luhmann (Communicating with Slip Boxes)
Here is a system of note-taking worthy of study!
You will find a detailed treatment of Zettelkasten in How to Take Smart Notes by the estimable, and often hilarious, Sönke Ahrens. You will also find links to other great resources at the end of this article, but for now fellow adventurer, let us quickly explore they key elements of Luhmann's system and what a modern version might look like.
NOTE: You will see the word "content" used quite often in what follows. Whenever you see that word, think books, audiobooks, articles, YouTube videos, podcasts, art and anything else you consume that has the potential to challenge or inspire you.
Zettelkasten consists of the following key elements:
Start by carefully selecting what you consume based on what is of interest. I use the following sources in order of importance:
Put simply, establish your own trusted sources for new content — beginning with the references and links in what you are currently reading, watching or listening to.
Be selective and move on if the content fails to generate ideas or challenge your existing mental models. There is simply too much for one human to consume in many lifetimes, so cut your losses and move on.
As you read a book, watch that next great TED talk on YouTube or listen to an episode of your favorite podcast, jot down notes in your own words about the things that interest you most. Capture page numbers or times of occurrence to revisit the source later.
Learning is initiated by the process of putting into words what inspires you most about the content you consume. Understanding is aided by seeing your words on paper (or on screen) and being confronted with their meaning. A quick cycle of thinking, re-reading and rephrasing then occurs until you are satisfied enough to move on. This process may be awkward at first, but improvement comes quickly with practice. Writing truly is the medium of understanding.
It is impossible to think without writing; at least it is impossible in any sophisticated or networked fashion.
— Niklas Luhmann (Communicating with Slip Boxes)
Most people simply highlight passages in a book and move on. Ask yourself how much of what you highlight is ever used or even seen again? Highlighting is great for quotes and references, but it does little to stimulate the mind.
Luhmann had two slip-boxes: a reference slip-box for bibliographical information and an idea slip-box for collecting notes and generating ideas from them. Each reference or note was written on one slip of A6-sized paper and numbered. You will often see the words "slip" and "card" used interchangeably to describe the contents of Luhmann's slip-box, but since he used the thinnest paper to fit as many as possible in each drawer, I will use the word slip.
The first of Luhmann's slip-boxes contained bibliographical information or references for each book, article or other content from which he generated notes. Ahrens refers to the kinds of notes generated while consuming content as "literature notes." Each slip in the reference slip-box contained bibliographical information on one side and brief literature notes about its contents on the other side. Thanks to the simple numbering system, any note in the idea slip-box could easily reference its source simply by denoting the reference's number.
I have only consumed about 10 sources at this point in my own journey, four of which are books. I have already found that each successive book contains markedly less literature notes than the one before it. Although purely empirical, I do believe there is an important effect to be aware of here. You will tend to generate more notes early on than you will later. You will also tend to generate more lengthy notes early on than you will later. I think this is primary due to the deepening of your skill as a writer combined with the weight of your existing slip-box. You become a better writer over time, notes become more efficient and the slip-box alter ego forces you to be ever more selective as it grows and matures.
Luhmann would routinely review literature notes, consider them in relation to his interests and compare them to what already existed in his second slip-box, the idea slip-box. The thoughts generated from this process would yield new notes, each of which he would write on one side of a slip, number and place in his idea slip-box. Each note in the idea slip-box contained an insight or comment related to one idea. Luhmann would add additional slips as necessary to extend an idea. A mature slip-box forces you to determine what is relevant and what is not.
Ahrens refers to the notes in your idea slip-box as "permanent notes." These kinds of notes are ideas in writing that no longer require the original context. Their ultimate relevance is the concern of future thinking. Each note should be treated as a completed manuscript that others might read. Keep them concise, grammatically correct, polished and coherent. Always strive for simplicity and directness.
The Zettelkasten numbering system, essentially alternating numbers and letters, supports both the linear continuation of existing notes and the arbitrary branching of notes. Luhmann himself describes it best.
We do not need to add notes at the end, but we can connect them anywhere—even to a particular word in the middle of a continuous text. A slip with number 57/12 can then be continued with 57/13, etc. At the same time it can be supplemented at a certain word or thought by 57/12a or 57/12b, etc. Internally, this slip can be complemented by 57/12a1, etc. On the page itself I use red letters or numbers in order to mark the place of connection. There can be several places of connection on a slip. In this way, a kind of internal growth (Wachstum nach innen) is made possible, depending on what kind of material for thought occurs.
— Niklas Luhmann (Communicating with Slip Boxes)
Luhmann concedes that the resulting structure of notes is a fragmented form of the original text, but the numbering system allows one to easily reconstruct it. In fact, this is quite an understatement, because you can follow any branch through the sequence of notes to construct a linear argument worthy of publishing. What you have gained is an infinitely flexible system that on one hand allows the mind to add and branch areas of thought without constraint and on the other hand construct linear patterns of thought for publishing articles, books and other media.
We have discussed notes and numbering, but what about links between notes? The slip-box system has three kinds of links:
One note may reference another simply by including its number. Likewise, it is common practice to include a backlink such that each note references the other; in essence, a bidirectional relationship. Luhmann himself did this and encouraged it. The practice of adding links has a nice side effect. Not only do you link ideas, you enrich each note in the act of linking them.
Since slips with bibliographical information in the reference slip-box are also numbered, they can easily be linked from notes in the idea slip-box in much the same way. Luhmann encouraged linking notes back to the books, articles and other sources that inspired them. Not only do such links become useful when publishing our own content with appropriate references, but one of the ways we tend to remember certain topics is by author name.
You will then not only be able to determine after some time what you actually read and what you only noted to prepare reading, but you can also add numbered links to the notes, which are based on this work or were suggested by it. This proves to be helpful because our own memory — others will have similar experiences to mine — works in part with key words and in part with author’s names.
— Niklas Luhmann (Communicating with Slip Boxes)
Luhmann created an index for his idea slip-box on slips that contained keywords linked to notes. He limited each keyword to one or two notes, because those notes served as entry points, main topics or lines of inquiry to his network of related ideas instead of a mere archive of individual, disconnected notes. It was the relationships within that held the most value for their ability to aid in thinking and learning.
Considering the absence of a systematic order, we must regulate the process of rediscovery of notes, for we cannot rely on our memory of numbers. The alternation of numbers and alphabetic characters in numbering the slips helps memory and is an optical aid when we search for them, but it is insufficient. Therefore we need a register of keywords that we constantly update.
— Niklas Luhmann (Communicating with Slip Boxes)
The topics linked from your own index are subject to change as your understanding deepens. Topics or lines of inquiry are not set in stone and evolve with the introduction of new, related notes. The concept of keywords as a way to rediscover notes, as opposed to simply categorizing or archiving them, may be new to you. A couple of examples from my own index may aid understanding:
When forming your own index, just ask yourself this question: "How can I best describe this new line of thinking in a simple phrase such that I am more likely to stumble across it in the future?"
Now that we have established the basic elements of Luhmann's slip-box system, how do we create our own? Using wood boxes and slips of paper is certainly one way to go, but we can further simplify the process and provide more redundancy with modern tools. I suggest the following enhancements based on my limited experience so far:
I personally switched to eBooks years ago. The Kindle Reader app and the Apple Books app allow me to highlight passages and to write literature notes in the digital equivalent of a book's margins. I have a system for highlighting that takes advantage of coloring to denote regular highlights, quotes, principles/definitions and notable people I would like to research further. Here are some examples from The Obstacle Is the Way by Ryan Holiday and Atomic Habits by James Clear with Kindle colors and any notes I took attached to the highlight:
I use Readwise (Clippings is an alternative) to export my highlights and notes. Readwise formats them nicely and provides both web and mobile apps. My two favorite features are 1) export to popular notes apps like Evernote or Notion and 2) a review feature that aids learning and memory of what you read.
For articles found on the web, you can use the web clipping features of a tool like Evernote, Notion or Instapaper. Once "clipped" into one of these apps, you can highlight and comment directly on a local copy of the article.
For an actual book, a video or audio, you can take literature notes directly in your favorite note-taking app. Be sure to capture the page number or time for each note, so that you can easily jump to it again in context or reference it directly in your own published works.
What about permanent notes? I personally use Obsidian, because it is based on simple markdown files that I control locally. I keep my slip-box notes in a folder that is synched to the cloud automatically. Roam is another excellent alternative. Both of these tools, and others like them, provide an embarrassment of riches for managing linked notes.
So what exactly am I referring to when I say "idea?" Some typical examples include:
Consuming content and taking notes will generate A LOT of ideas — the ideas will hit you at all hours of the day. If you have ever wondered why the best ideas seem to jolt you awake at night or suddenly occur to you while performing daily ablutions, it is because your brain thrives on answering questions and solving problems. It does this on a background thread and pops out new ideas and solutions when formed.
Be ready to catch those little nuggets of gold, for It is well nigh a criminal act to let one of them slip through the proverbial shower drain. Set up a simple system for jotting down each idea for review later when you have the time, and if you are otherwise indisposed, repeat the idea over and over in your mind or out loud to imprint it in short-term memory. Most of us have a so called "smart" phone literally attached to our foreheads, so it won't be long before you can toss that new idea into your digital inbox for later review.
I have always been a big fan of productivity systems like Getting Things Done, but the concept of an inbox never really stuck until I recently came across Ali Abdaal and his Resonance Calendar for capturing content that resonates. Doing this in an almost frictionless way is possible with tools like Notion. Shu Omi explains how to extend the Resonance Calendar to Roam. Whether you use Notion, Evernote, Roam, another tool or even a combination of tools, make the process as frictionless as possible to capture ideas and content for later review.
Don't ever forget another idea!
As discussed earlier, a key feature of Luhmann's Zettelkasten system is the linking and back linking of notes. While Luhmann used a simple, but powerful numbering system, tools like Obsidian and Roam make this super easy to do. In fact, even though some people may still prefer and advocate for a numbering system, it is no longer strictly necessary. That makes the Zettelkasten system even easier to achieve!
Let us take a look at what some of my notes look like in Obsidian.
The following note is in my reference slip-box. It contains bibliographical information and literature notes. Just a few of the literature notes are shown here. Notice how I link each note to a page number. The location links actually open up the Kindle Reader app and take me directly to the source in context.
The following excerpt is from my index. This is the un-rendered form in markdown so you can see what a link looks like (the double brackets) between two notes.
The Kaizen link from the index is associated with the following permanent note. The hash symbol denotes a tag. You can see a link to the reference from which the note was inspires. It is common to see many links in a gateway note like this. For example, this one has fifteen links. When I think about small change, I currently enter this gateway note first and then follow the internal links.
This next note is one of the internal links from the gateway note. Although not always the case, most of my internal notes link to other internal notes as seen here.
There is a gravity that builds with these notes and the links between them. The further I develop a line of inquiry, the stronger the pull on my mind — a mind that enjoys its time here. Each note is written by a previous version of myself, and I am already delighting in insights I had forgotten. It is alarming how quickly thoughts fade but also comforting that they are all still here waiting to be discovered, sometimes in the same context while at other times offering new insights into to new contexts. I am beginning to understand how Luhmann's alter ego developed over time. I hear the faint sounds of my own communication partner forming even now.
Set aside time to read and write literature notes for at least half an hour each day. Set aside additional time each day, week or after completing a particular piece of content (e.g., book, article, video, podcast) to transfer your literature notes into a bibliographical reference and permanent notes. By consuming what interests and excites you, these habits are more likely to stick because you will crave them. This is the fuel of future ideas, so do not neglect it.
Treat your slip-boxes like prized gardens. Tend them regularly, prune the weeds and keep out unwanted guests. I look forward to a not too distant future where my own alter ego forms and begins to provide its own interesting insights and arguments. Until then, I will continue adding notes, linking them, thinking, learning and understanding. I hope you do too, fellow adventurer!
My Review of How to Take Smart Notes by Sönke Ahrens
Communicating with Slip Boxes by Niklas Luhmann
Niklas Luhmann's Card Index: Thinking Tool, Communication Partner, Publication Machine by Johannes F.K. Schmidt
Masters of Creative Note-Taking: Luhmann and Da Vinci by Tiago Forte
Zettelkasten for Writers by Eva Deverell
Zettelkasten — How One German Scholar Was So Freakishly Productive by David B. Clear
The Zettelkasten Method by Abram Demski
