A zettelkasten consists of many individual notes with ideas and other short pieces of information that are taken down as they occur or are acquired. The notes are numbered hierarchically, so that new notes may be inserted at the appropriate place, and contain metadata to allow the note-taker to associate notes with each other. For example, notes may contain tags that describe key aspects of the note, and they may reference other notes. The numbering, metadata, format and structure of the notes is subject to variation depending on the specific method employed.
A zettelkasten may be created and used in a digital format, sometimes using personal knowledge management software. But it can be and has long been done on paper using index cards.
The method not only allows a researcher to store and retrieve information related to their research, but also intends to enhance creativity. Cross-referencing notes through tags allows the researcher to perceive connections and relationships between individual items of information that may not be apparent in isolation.
In the form of paper index cards in boxes, the zettelkasten has long been used by individual researchers and by organizations to manage information, including the specialized form of the library catalog. Coming from a commonplace book tradition, Conrad Gessner (1516–1565) invented his own method of organization in which the individual notes could be rearranged at any time.
The first early modern card index was designed by 17th-century English inventor Thomas Harrison (c. 1640s). Harrison's manuscript on The Ark of Studies (Arca studiorum) describes a small cabinet that allows users to excerpt books and file their notes in a specific order by attaching pieces of paper to metal hooks labeled by subject headings. Harrison's system was edited and improved by Vincent Placcius in his well-known handbook on excerpting methods (De arte excerpendi, 1689). Librarian and mathematician Gottfried Wilhelm Leibniz was known to have relied on Harrison’s invention in at least some of his research.
In 1767, Carl Linnaeus used "little paper slips of a standard size" to record information for his research. Over 1,000 of Linnaeus's precursors to the modern index card containing information collected from books and other publications and measuring five by three inches are housed at the Linnean Society of London.
Later in his own commonplace, under the heading "My way of collecting materials for future writings" (translated), Johann Jacob Moser (1701–1785) described the algorithms with which he filled his "card boxes".
The 1794 idyll Leben des Quintus Fixlein by novelist Jean Paul is structured according to the zettelkasten in which the protagonist keeps his autobiography. Jean Paul ultimately assembled 12,000 paper scraps into his commonplace books over the course of his lifetime, but died in 1825, almost a century before the advent of standardized note cards or box systems which later made it easier to store and organize them.
Philosopher and intellectual historian Hans Blumenberg compiled more than 30,000 cards into his zettelkasten, which now occupy thirty-two conservation boxes at the German Literature Archive in Marbach. Blumenberg was inspired by the previous notetaking work and output of Georg Christoph Lichtenberg who used waste books or sudelbücher as he called them.
One researcher famous for his extensive use of the method was the sociologist Niklas Luhmann (1927–1998). Luhmann built up a zettelkasten of some 90,000 index cards for his research, and credited it for enabling his extraordinarily prolific writing (including over 70 books and 400 scholarly articles). He linked the cards together by assigning each a unique index number based on a branching hierarchy. These index cards were digitized and made available online in 2019. Luhmann described the zettelkasten as part of his research into systems theory in the essay "Kommunikation mit Zettelkästen".
Other well known zettelkasten users include Arno Schmidt, Walter Kempowski, Friedrich Kittler, and Aby Warburg, whose works along with those of Paul, Blumenberg, and Luhmann appeared in the 2013 exhibition "Zettelkästen. Machines of Fantasy" at the Museum of Modern Literature, Marbach am Neckar.
While not referred to specifically as zettelkasten by their non-German speaking users, there is a tradition of keeping similar notes in a commonplace book-like tradition in other countries. American comedians Phyllis Diller (with 52,000 3x5 inch index cards), Joan Rivers (over a million 3x5 inch index cards), Bob Hope (85,000 pages in files), and George Carlin (paper notes in folders) were known for keeping joke or gag files throughout their careers. They often compiled their notes from scraps of paper, receipts, laundry lists, and matchbooks which served the function of waste books. U.S. president Ronald Reagan kept punchlines for speeches in a card collection.
- Argument map
- Commonplace book
- Issue-based information system
- Personal knowledge base
- Personal knowledge management
- Personal wiki
- Reference management software
- Tag (metadata) § Knowledge tags
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