When I have some important problem I don’t know how to solve, I of course think about it all the time. Every now and again I’ll think of something that seems extra useful and write it down.

Eventually I’m looking at a whole page of good ideas, and some pattern jumps out at me. I attribute many of my best ideas to this process. Writing lets you work beyond the limits of your own brain, especially the constraints of working memory.

I think that is the primary value in a “second brain” - externalizing your thoughts and reasoning makes them stronger - though the productivity community may use the term to mean some other thing.