The Zettelkasten (slip box) method, pioneered by sociologist Niklas Luhmann who published over 70 books using it, is a note-taking system designed to generate new ideas through the connections between notes. Unlike hierarchical filing systems, the Zettelkasten treats every note as an atomic idea linked to other ideas, creating an emergent web of knowledge. This article explains the principles — atomicity, connectivity, and emergence — and provides practical guidance for implementing a digital Zettelkasten using modern tools.