Introductions to zettelkasten proliferate on the internet. Unfortunately, so do confusing, contradictory explanations of its most basic concepts. These explanations range from the misinformed to the intentionally obfuscated. A shame, since zettelkasten is both an elegant concept and an effective practice. But its effectiveness depends on your understanding of its core ideas and, crucially, your application of these ideas. So, I'd like to briefly introduce these concepts, as well as give some contrasts between zettelkasten and other note-taking systems in the hope that it clarifies the concept for some people.

Note-taking as a thinking tool has been around probably since the invention of cheap paper goods. The first card-based systems are several hundred years old, at a minimum. In reality, there are probably as many note-taking systems as there are people interested in note-taking. It is a very personal process. Like all writing, it often reveals more about yourself than about anything objective about note-taking. Into this history, Niklas Luhmann introduced his zettelkasten, his personal approach, adapted from earlier, similar systems. Luhmann's motivation for creating such a system was his desire to find a good research assistant: the time, money, and effort it would take to maintain such a person was just too great.^[1]

Although adapted from earlier, similar systems, Luhmann incorporated at least three new concepts into his note-taking that made his zettelkasten unique. Other people may have used variations of these ideas earlier than Luhmann, but as far as I can tell, Luhmann was the first person to combine all three into a whole united in purpose.

The most important of these three concepts is shared context. Other note-taking systems preferred to put thoughts to paper, refine them, and then file them away into some hierarchical order, like according to topic, for example. In other words, each note mostly stood on its own, within its own context. With Luhmann's zettelkasten, on the other hand, notes were expected to share some of this context between one another. In other words, notes were expected to respond in some way to the ideas found in other notes. Luhmann further realized that this shared context gave structure to his notes which eliminated the need to file things away into pre-defined categories or rigid topics. In fact, by linking his ideas in this way, he created a web of notes that grew over time, dense in some areas and sparse in others. These dense zones were a lot like topics in themselves, but they didn't start that way. Fortunately for Luhmann, since he would continue to use this system for the next 30 years, he recognized this feature of his zettelkasten as key to its usefulness.

His next innovation was to deploy fixed addresses for each note. Luhmann recognized that over 30 years his web of notes would grow wild and there would need to be some sort of ordering. Unlike a hierarchical address scheme where the relationship between notes is one of parent-child, Luhmann's scheme presents no such relationship. Instead, Luhmann's address scheme was based on how the context between notes was shared. In the event that two notes speak directly to one another, they share most of the address. The new note would 'extend' the previous note, like '1.1' and '1.1a'^[2]. Another example is a note with address '5.3a1b' arguing against a thought found in a note with address '5.3a1', each time alternating between numbers and letters as appropriate. The most important thing to understand is that address '5.3a1b' is not less important, less interesting, less general, or any other hierarchical relationship imaginable compared to '5.3a1a' or '5.3a1'. It is simply the second thought, which is in response to '5.3a1'.

Another case is when two notes are related but the new note does not necessarily speak directly to the previous note. In that case, Luhmann made the notes siblings: '1.1' and '1.2'. This includes notes which occur at any level of nesting, such as '1.1a1' and '1.1a2', or '5.3a1a' and '5.3a1b'. These notes relate ideas that themselves do not speak directly to one another but both refer back to a common idea. They are separate chains of ideas that can be followed independently and at will.

Finally, when two notes have nothing in common (which is especially common in the beginning), Luhmann gives each note a new address: '1.1' and '2.1'. It didn't matter to Luhmann that an idea was given the address '1.1' over another because, despite common cultural association, the number '1' signifies only an arbitrary starting address for ideas. In Luhmann's zettelkasten, it's common to find chains of ideas with a specific context which are nearly wholly unrelated to the original idea found at the top of the section.

This practice might seem strange and unproductive at first, but the question of where to file an idea, especially in the beginning of the practice when the abundance of notes is very low, can be confusing. We ascribe too much meaning to the ordinal aspect of numbers when we think to ourselves during our zettelkasten practice that an idea which is addressed with the number '1' must in some way be 'more general', 'more important', or in any way higher in some hierarchy of ideas than one addressed with the number '2' and so on. After a very short amount of time using your zettelkasten, you will begin to see easily where ideas fit into the whole: their addresses will become more obvious to you as you're creating notes until, eventually, the possibilities of so many addresses will become its own problem.

Notes in zettelkasten are more properly thought of as nodes in a network. Imagine the more or less linear shape of note cards in a box unfurling out of the box and into the third dimension, the connections between ideas being like bright strings illuminating a network. In such a network, no one node is privileged with being the first. The concept of first simply doesn't apply in this structure, no more than a cube has a first corner. Likewise, adjacent nodes do not suggest any parent-child hierarchy simply by their proximity. However, if we had to label such nodes, we could begin by selecting a node at random and assigning it an address such as '1.1'. From there, all other nodes could be assigned addresses according to the rules we just outlined. This process is no doubt subjective, but the purpose of zettelkasten is not to discover connections about ideas in the abstract, but rather about connections between your own ideas. It is therefore necessarily subjective.

Luhmann's address scheme also solves another problem, one common to most other note-taking systems, that of multiple possibilities for a note's location. For example, in a topic-hierarchy system, there comes a point where a note could conceivably be filed in multiple locations. This is especially common for notes relating to interdisciplinary topics, such as the history of philosophy, or the philosophy of mathematics, for example. Anyone who has ever tried to organize files on a computer according to topics or filetypes will know that this is a common problem. But by giving each note a fixed address, Luhmann ensured that whenever he found a note which spoke to multiple contexts he could link them together. That is, Luhmann would reference the remote note on the new note with a bit of helpful context for why the two ideas were related. This means that from one note, Luhmann could pursue the idea in multiple directions, or equivalently, that he could arrive at such a note from multiple contexts. This elegantly solves the problem at hand. For Luhmann, a note on the topic of the history of philosophy could appear anywhere in his zettelkasten, provided there was some context for this idea already established.

Luhmann's last innovation I want to talk about was his keyword index. Recognizing that ideas which were not indexed were likely to be lost in the multiple tens of thousands of notes that he worked with, he maintained a separate section of his zettelkasten just for indexing keywords and a few locations where that idea could be found. Crucially, however, his index was never 'complete', meaning he intentionally limited himself to no more than four addresses per keyword. With so many notes, this approach was very likely to miss a few, but Luhmann relied on his internal linking to help him navigate his zettelkasten once he entered in through one of these addresses. This created an opportunity for surprise, as Luhmann puts it, ensuring that even Luhmann himself wouldn't necessarily know the outcome of his query. This highlights and reiterates an important point about zettelkasten: it is not a knowledge repository; therefore, Luhmann was unconcerned with the 'completeness' of his index. Instead, it was a tool for generating novel connections between ideas. By being forced to essentially enter his zettelkasten blind in search of a concept, it ensured his mind was primed to make connections and those connections were essentially random.

Although I've attempted to distill Luhmann's practices here, I think it's worth emphasizing again that the practice of note-taking is subjective and what worked for Luhmann may not work for you. However, by dissecting some of the rationality behind Luhmann's famously opaque methods, I hope to empower newcomers to make their own choices with regards to practices and habits. Luhmann credited his prolificacy to his Zettelkasten, but I believe his comments were misunderstood. In reality, Luhmann spent an enormous amount of time writing and reading on most days. Under those conditions, Zettelkasten or not, almost anyone could achieve the success that Luhmann did. Instead of focusing on copying his methods and hoping that success follows, I think it's better to work hard and focus on making his ideas work for you.

Having spent time together with my zettelkasten, both making and unmaking it, I've learned that understanding these three big ideas is the most important for being able to effectively use your zettelkasten, and crucially, to answer your own questions when confusions and ambiguities inevitably arise, especially in the beginning. Zettelkasten can be very confusing for newcomers. Between the terminology, the tools, the workflows, and the necessity of each expositor saying something unique about the practice, there is little room left for explaining and exploring the foundational ideas of Niklas Luhmann in ways that empower the new practitioner to dive headfirst into it which is, after all, the point of the practice. To be sure, there is much more that could be said, just on Luhmann's ideas alone. However, by developing an understanding of the thoughts behind the key ideas of zettelkasten, you will be in a much better position to find what works for you on your zettelkasten journey.

Further Reading

• Luhmann, Niklas. “Communicating with Slip Boxes.” Two Essays by Niklas Luhmann.
• Sascha. “English Translation of All Notes on Zettelkasten by Luhmann.” Zettelkasten.
• Doto, Bob. A System for Writing. New Old Traditions, 2024.

When it comes to organizing books, traditional systems like the Dewey Decimal system are functional but uninspiring. What if there were a system that not only organized books but also encoded information about their subjects into the classification numbers themselves? How might libraries change as a result of the mass adoption of such a system? And how might our relationships with knowledge and access to information evolve?

Meet prime numbers: counting numbers (1, 2, 3, …) that can only be divided evenly by one and themselves. These are the key elements of such a system because every counting number can be uniquely expressed as the product of prime numbers, like how 10 is 2 times 5 or 21 is 3 times 7. Here’s a playful idea: prime numbers could be used as the basis for a classification system where subjects are associated with prime numbers. For illustration, imagine Art is 3 and History is 5. Then combinations of subjects could be represented as products of prime. In this system, a book about Art history would be classified as 15 since it’s 3 (Art) times 5 (History). Imagine browsing the “141” section of your local library or bookstore. Knowing that 141 breaks down as 3 (Art) times 47 (Psychology), you immediately grasp the subject nature of the books in front of you.

While this system is simple and effective, it has its challenges. Universal truths about numbers make a universal classification system tempting. But deciding which subjects are “prime” and assigning prime numbers to them would require global consensus. Standardization breeds uniformity, and we must be careful not to lose the diversity of experience that makes us human. Fortunately, this system’s flexibility could uphold that diversity. Adding new subjects is as easy as assigning the next prime number, and with infinitely many primes, there’s no reason to hold back.

However, larger prime numbers become less manageable, increasing our reliance on digital technology to navigate knowledge. Without an app, most librarians wouldn’t generate classification codes, and most users wouldn’t decode them. What might seem like an intrusion of technology into a sacred space today may become an indispensable tool tomorrow, much like digital computing.

With advancing medical and cybernetic technology, the ability to comprehend and manipulate large numbers could soon be within reach. Librarians of the future might classify millions of books instantly. In fact, the logical underpinnings of this system could become essential. Whereas understanding that “724.1” means “Baroque and Renaissance Architecture” in the Dewey Decimal system requires memorization, future systems could synthesize such facts from simpler “prime” subjects. This flexibility could lead to fascinating and complex applications.

Imagine cybernetic librarians locating books from a list of subject interests, logically combining them to explore niche topics further. As readers finish books, they could submit feedback on relevant subjects, which algorithmic classification programs would use to continuously update our understanding of what a book is about. The applications are nearly unlimited. Theorems about prime numbers could become practical tools in library science. Just like a laptop can run any program, a classification system based on these computational fundamentals might one day make the library’s system a powerful computer itself.

As we rely more on technology to navigate our confusing world, our systems inform us and guide us to new information. While this prime number system is not ready for mass adoption, it offers a fascinating vision of the future of knowledge and its organization. Yet it also raises questions about how technology shapes our understanding of reality through the interfaces it demands. Will we choose to “update” ourselves to adapt to these knowledge interfaces, remembering that all interfaces decide what to include and exclude? What will we give up for the convenience of such a system? And how will the fundamental organization of our knowledge change in response?