Commonplace books were the predecessors to PKM or the concept of a „Second Brain“

Commonplace books (or commonplaces) are a way to compile knowledge, usually by writing information into books. They have been kept from antiquity, and were kept particularly during the Renaissance and in the nineteenth century. - Wikipedia

Long before there were digital note-taking tools intellectuals had their own personal knowledge management system in the way of a personal book with them.

Nowadays it seems we rely too much on digital tools without any curation due to search functions, Google or AI finding or even creating any conceivable piece of knowledge on demand. - see Why is there a need for a personal knowledge management system? Why can’t I just use Google?

But in contrast to almost infinite information on the internet, In your personal knowledge management system, everything should be high quality and trusted knowledge.