Zettelkasten as a Family Resemblance Category

In contemporary English-language Zettelkasten and personal knowledge management discourse, Zettelkasten functions as a family-resemblance category organized around central exemplars. Niklas Luhmann's two collections and associated practice supply the primary historical exemplars.¹ Sönke Ahrens supplies an influential contemporary interpretation of the workflow and the terminology from which its note categories can be reconstructed. Systems are understood as more or less closely related to these historical and interpretive anchors along contextually relevant dimensions.

This shared field of resemblance is sufficient to recognize central cases, compare implementations, identify historical relationships, and conduct intelligible disputes about peripheral cases. It does not follow—and I do not claim—that every participant assigns every possible object the same yes-or-no classification.

Historical exemplars, interpretation, and implementation

Luhmann, Ahrens, and contemporary software play different roles in this account.

• Luhmann supplies the primary historical exemplars: two actual collections, their organization, identifiers, references, indexes, and use in sustained intellectual production.
• Ahrens supplies an influential contemporary interpretation: an account of a workflow involving fleeting, literature, permanent, main, and project notes. Ahrens's terminology is scattered and not presented as a complete taxonomy. From Fleeting Notes to Project Notes reconstructs the relations among these categories.
• The Archive, zkn3, Obsidian, Logseq, Zettlr, Roam Research, and similar programs supply implementation cases: materially different systems that can occupy some or all of the roles abstracted from Luhmann and Ahrens.

This tripartite distinction explains why a historical argument about Luhmann's collections, a terminological argument about Ahrens, and a practical comparison of software belong to the same discussion without reducing to one another.

Dimensions of resemblance

Relevant dimensions include:

• persistent notes intended for later use;
• individually addressable or retrievable notes;
• relations among notes;
• organization supporting navigation and rediscovery;
• continued elaboration of an existing collection;
• use of the collection to develop thought and produce writing;
• a workflow connecting capture, literature, permanent notes, and projects.

These dimensions are not proposed as jointly necessary and sufficient conditions. They describe respects in which systems resemble the central exemplars. One system may closely resemble Luhmann's collections in its identifiers and linking structure while resembling Ahrens's account in its workflow. Another may preserve the intellectual functions while replacing the physical and organizational mechanisms.

On this account, the category exhibits prototype effects: some cases are treated as more central or representative than others. This descriptive claim does not require the stronger psychological hypothesis that speakers represent the category by one abstract prototype. Here exemplar denotes a historically given reference case, not a claim about how speakers store or retrieve categories. The category itself is connected by overlapping similarities.

Relevant resemblance is determined by the context of comparison. A box of blank cards resembles Luhmann's collections materially but not as an intellectual practice. A digital collection of linked, persistent notes may resemble them structurally and functionally while lacking paper cards, Folgezettel, and wooden cabinets.

Software comparisons as evidence of common understanding

The intelligibility of comparing software implementations is evidence of a common understanding. Consider statements such as:

• “The Archive is a Zettelkasten application.”
• “Obsidian can be used as a Zettelkasten.”
• “zkn3 is closer to a traditional card index.”

If Zettelkasten supplied no shared basis for comparison, these statements would be arbitrary juxtapositions. They are not. Readers understand both the comparisons and the grounds on which someone might dispute them.

Disagreement remains possible. Someone may regard a program as too dependent on folders, search, backlinks, or graph navigation; someone else may regard those differences as changes of implementation rather than changes of kind. Such disputes concern the relevance and weight of similarities. Their intelligibility presupposes shared central cases even when their conclusions differ.

Common understanding without a Boolean test

Common understanding does not require a generally accepted predicate $Z(x)$ that classifies every possible object $x$ as either a Zettelkasten or not a Zettelkasten. It may instead consist in shared competence concerning central examples, relevant comparisons, ordinary inferences, and clear nonexamples, while leaving some cases indeterminate or disputed.

The logical point is:

1. A generally accepted necessary-and-sufficient criterion would determine a common extension of the term.
2. Shared understanding can exist without such a criterion.
3. Therefore, the absence of an agreed criterion does not imply the absence of shared understanding.

In symbols, if $B$ means that a discourse community accepts a common yes-or-no criterion and $U$ means that it has a shared practical and inferential understanding, then

$$ U \not\Rightarrow B \qquad\text{and consequently}\qquad \neg B \not\Rightarrow \neg U. $$

The absence of an agreed Boolean criterion establishes only that no such criterion is agreed; it does not determine how much agreement exists about either central or boundary cases.

Scope and polysemy

The German word Zettelkasten has ordinary, historical, commercial, and technical uses. It can refer literally to a box containing slips, to particular physical collections, to a method, or to software. This polysemy does not prevent contextual understanding. A specialized discussion of Zettelkasten practice need not adopt a single definition covering every historical use of the word.

Claims intended to cover all Zettelkasten systems may require an explicit scope. That requirement is compatible with common understanding in ordinary discussion. It distinguishes the lack of a universal definition from the lack of a shared subject.

Wittgenstein's Zettel

Wittgenstein supplies a figure worth recalling. In the Philosophical Investigations §293, everyone has a box, everyone calls the thing in it a “beetle,” and no one can look into anyone else’s box. The boxes might contain different things, or nothing at all, without affecting the word’s use. The private object drops out of the language-game.

There is a pertinent historical irony. Wittgenstein's Zettel, first published in 1967 and edited by G. E. M. Anscombe and G. H. von Wright, was made from fragments Wittgenstein had left in a box-file, most of them cut from his typescripts. The editors report that some other fragments were handwritten, apparently to supplement remarks on particular subjects already preserved in the box. Peter Geach arranged the cuttings, some of which had been clipped together while others remained loose.

The philosopher whose account of family resemblance is routinely used to explain classification without necessary-and-sufficient conditions also left the fragments later published as Zettel in a box-file. This does not make Wittgenstein's box identical to Luhmann's collections or establish a further necessary condition for being a Zettelkasten. The juxtaposition illustrates the opposite: the meanings of Zettel and Zettelkasten, and the classification of related practices, are supplied by use and context, not by one Boolean test covering every historical object and every application of the words.

Notes

1. The centrality of Luhmann is a fact about the current discourse rather than about the practice. A longer historical scope would include different cases, going back at least to Gessner (1548) and taking in the commonplace-book tradition (cf. Blair 2010). Chris Aldrich has documented that longer tradition and has argued that current usage of Zettelkasten is narrower than the historical record warrants (Aldrich 2022, 2023).

References

• Ahrens, Sönke. How to Take Smart Notes. 2017; second edition, 2022.
• Aldrich, Chris. “Zettelkasten Overreach.” BoffoSocko, February 5, 2022. https://boffosocko.com/2022/02/05/zettelkasten-overreach/
• Aldrich, Chris. “A Note on the Cargo Cult of Zettelkasten.” BoffoSocko, February 3, 2023. https://boffosocko.com/2023/02/03/a-note-on-the-cargo-cult-of-zettelkasten/
• Blair, Ann M. Too Much to Know: Managing Scholarly Information before the Modern Age. New Haven: Yale University Press, 2010.
• Gessner, Konrad. Pandectarum sive partitionum universalium libri XXI. Zurich: Christoph Froschauer, 1548.
• Lengyel, F. From Fleeting Notes to Project Notes.
• Rosch, Eleanor, and Carolyn B. Mervis. “Family Resemblances: Studies in the Internal Structure of Categories.” Cognitive Psychology 7, no. 4 (1975): 573–605.
• Schmidt, Johannes F. K. “Der Nachlass Niklas Luhmanns – eine erste Sichtung: Zettelkasten und Manuskripte.” Soziale Systeme 19, no. 1 (2013): 167–183.
• Wittgenstein, Ludwig. Philosophical Investigations. 4th ed. Edited by P. M. S. Hacker and Joachim Schulte; translated by G. E. M. Anscombe, P. M. S. Hacker, and Schulte. Wiley-Blackwell, 2009. §§65–71, 293.
• Wittgenstein, Ludwig. Zettel. Edited by G. E. M. Anscombe and G. H. von Wright; translated by Anscombe. Oxford: Basil Blackwell, 1967. See the editors' preface.