A notebook carried everywhere records which observations survived the inconvenience of writing them down.

A notebook is a filter as much as a record. Stopping, finding the page and writing costs enough that only thoughts with some weight behind them get through, while the rest evaporate on the way, which is mostly fine. What ends up on the page is not everything you noticed but everything that insisted. Friction does the sorting for you.

Carry something to write on for a fortnight and see what survives the effort. Do not tidy the entries or judge them as you go. Reading back a month later shows what you actually think about, which is rarely what you assumed.