It's probably apocryphal, but Richard Feynman was said to keep two lists, a "problem" list and a "solution" list.

The problem list contained all the questions and challenges Feynman was curious about, from profound scientific inquiries to practical problems or simply things he didn't understand but wanted to explore. The "solution" list contained some broad subset of the tools, concepts, techniques, and ideas Feynman had mastered or understood. Then he'd just occasionally cross check them against one another.

I've always been fascinated by this idea, and I'm curious to know if anyone contemporary has tried something like this and found it useful.