Niklas Luhmann

Zettelkasten originator; second-brain ancestor.

German sociologist and professor at the University of Bielefeld until his death in 1998, Luhmann authored over 70 books and 400 articles across a 30-year career. He credited his output to a physical Zettelkasten of roughly 90,000 index cards organised by emergent links rather than fixed categories — a method now widely adapted in digital note-taking tools and PKM practice.