Even absent any formal system of organization, I've found index cards to be an incredible knowledge-management life-hack. Their size strikes a perfect balance:

Even absent any formal system of organization, I've found index cards to be an incredible knowledge-management life-hack. Their size strikes a perfect balance: use a whole card for a single idea, or use fine print to scribble in lots of details (flip over, free 2x real estate). They're trivially "composable", using paper clips or binder clips, for clustering together related notes and tasks; but they can be trivially separated again to pass to someone else, or can be laid out on a table to see them all at once (try that with a notebook!). If you finish a card, you can tear it in half, which is satisfying. You can trivially sort to prioritize, and they can fit in your pockets. The biggest downside is that it's easy for cards to get lost in stacks: "out of sight, out of mind". In that sense, sticky-notes do still have an advantage; and while notebooks have the same problem, at least there's a degree of object permanence (page 10 of Notebook #7 will never leave its spot). Nonetheless: index cards are an under-appreciated practical tool for off-boarding memory and capturing ideas.