For those not enlightened by the German title, "Zettelkasten", or "slip-card box", is an index-card based information capture and organisation system created by the late Geman sociologist Niklas Luhmann.
He is famous for having written his books on basis of his Zettelkasten but I doubt he invented it. It could well be he thought so himself though after having lost/omitted the external reference on that index card of his named Zettelkasten.
