Karl Kraus, one of the cult figures of twentieth-century literature and criticism, wrote:
"Anyone who writes in order to display education (Bildung) must have memory; and then he is merely an ass. If he also uses the scientific disciplines or the card index (Zettelkasten), he is also a fraud" [Die Fackel , Heft 279-80 (1909)].
In this spirit he castigated Alexander Harden as "an enemy of the spirit that was fed by a small mind with a large card index," taking up what appears to have been a common criticism of the author, who because of his style that relied overly much on quotations [Die Fackel, Heft 360-62 (1912)]. In "Desperando or the Language of the Future," he rendered Harden's phrase "to dream of the teachings of ancient history under the sail of the sun" as "becoming sea-sick from a card index" [Die Fackel, Heft 251-2 (1908)]. And in his entries on Freiherr von Berger (Alfred von Berger), a friend and defender of Harden's, he accused him of naivité. Von Berger would explain the miraculous phenomenon of Harden's "apparently truly inexhaustible knowledge that he has whenever needed ready at hand" by "denying the existence of card indexes" altogether. While von Berger "never believed" in Harden's card index, the real question for Kraus was whether "Harden possesses anything apart from the card index, which he does not possess" [Die Fackel, Heft 311-12 (1910)]. And in another context, he sarcastically admitted that Harden might not have had an external card index, but that his brain was nothing but a card index. His mind had been reduced to the tool he did not use.
While Kraus was talking in the quoted passages about the use of index cards in the creation of literary texts, his contempt for the application of such an instrument seems to have been pervasive. It characterized for him a "specific" kind of memory that characterizes the "daily writer" (Tagschreiber) and consists "of names and sayings one has heard, of mis-heard [i.e. misunderstood] judgments and badly-read reports, of concepts and histories without context, of facts seen distortedly, of fifty fashionable expressions, and of the additional feature of one's own fragmentary knowledge (Fetzenwissen)" [Die Fackel, Heft 230-31 (July 15, 1907)]. Kraus believed that this kind of "jargonist knowledge" was favored by the schools of his time, which mistake true education with worthless memorial ballast and the knowledge of parakeets.
Kraus was neither the first nor the only thinker who criticized the card index in this way. In fact, it is one of the standard devices in criticizing scholarly books. If an author adduces too much documentation and does not reflect enough on the materials, he is likely to be criticized for having emptied his card index: The book reads like a dumped card index ("wie ein ausgeschütteter Zettelkasten"). My Doktorvater once criticized a book by saying that he could still see in the text where one 3x5 card ended and the other one began. It struck me as an appropriate - and devastating - criticism of the author.
Luhmann's books, for instance, are not completely free of this "feature" either. But someone's criticism is someone else's preference, just as some program's bugs become features of the program. Much of what passes for "non-linearity" in scholarly (and other kinds of) literature belongs here. We should be careful that we do not become our own tools.
My view: "non-linearity" may be good for note-taking process, but it is not good for the presentation of the material. And yes, I agree with Kraus, that a card index is a tool with inherent dangers. But the same may be said about any sharp instrument.
I could not have written this post without access to the electronic version of The Fackel by the Austrian Academy. See Die Fackel.