Categories create the illusion of organisation. Ideas stored by subject are easy to retrieve, but retrieval is not the hard part. A note about process design and a note about human psychology might be related, but they will not meet just because both were filed correctly. The hard part is making connections between things that do not obviously belong together. Note titles are APIs for thought is what makes those connections usable, a good title lets an idea travel into another note’s argument without re-explanation.