An interesting thought often arrives at an inconvenient time. You are between tasks, about to sleep, or halfway through something else. The temptation is either to drop everything or assume you will remember it later.
An idea notebook offers a third option. Capture enough to recognize the thought again without pretending you have already solved it. A title, a question, and one possible next step can be enough.
Separate the idea from the problem it is meant to address. “Make an app” is a possible solution. “People keep losing track of which articles they wanted to read” is a problem you can investigate. They may lead to different decisions.
GOV.UK's guidance on understanding user needs recommends learning what people are trying to do and how they currently do it. Its service-design setting is broader than a personal notebook, but the principle can help you turn an assumption into a question.
When you revisit a note, ask whether it still interests you and what small action could teach you something. A sample paragraph, a sketch, or a conversation with a likely user may be more useful than another hour refining the title.
Do not force every note to become a project. Some thoughts are useful because they connect to something later. Others can be crossed out. A notebook is a place to preserve possibilities, not a ledger of obligations.
Try three labels: explore, try, and leave for now. Move one idea forward only when you have a reason. The notebook becomes easier to return to when opening it does not mean agreeing to finish everything inside.
Further reading: GOV.UK: learning about users and their needs