For those in the back: Niklas Luhmann did not invent the Zettelkasten.
If this is something you have a penchant to forget there are bumper stickers now.

White bumper sticker with a cartoon image of Luhmann  with text that reads "Niklas Luhmann did not invent the zettelkasten"
Or if you need a mug for drinking your coffee as you process your permanent notes…
White coffee mug with cartoon image of Niklas Luhmann staring at the viewer