"One of the things that made his Zettelkasten or slip box (or note card file) so intriguing to the larger (German) public was a 1981 paper, entitled "Kommunikation mit Zettelkästen. Ein Erfahrungsbericht" (Communication with Index Card Systems. An Empirical Account. [...]) Luhmann claimed that his file was something of a collaborator in his work, a largely independent partner in his research and writing. It might have started out as a mere apprentice when Luhmann was still studying himself (in 1951), but after thirty years of having been fed information by the human collaborator it had acquired the ability of surprising him again an again. Since the ability of genuinely surprising one another is an essential characteristic of genuine communication, he argued that there was actually communication going on between himself and his partner in theory.
Luhmann also described his system as his secondary memory (Zweitgedächtnis), alter ego, or his reading memory or (Lesegedächtnis)."