Why keep a commonplace book
Keep other people's lines so they can return. Collecting without return is only storage.
You type why keep a commonplace book because the habit sounds old, and you want a reason that is not nostalgia.
You keep one so other people's lines can return. That is the whole motive.
The unit is an item: the text, its source, and the date you collected it. You are its keeper, not its author. A short annotation about why it stayed is enough. Skip the annotation if the line is enough. Later, once you have half-forgotten it, the item returns as a recollection.
Without return, collecting is only storage. Without a source, return is only a blur. The book is useful when a line you almost lost comes back with its place still attached.
A journal keeps your day. A vault keeps files. A commonplace book keeps other people's words for the day you need them again.
If that is the search, continue here: How to keep a commonplace book.
Opuss is that notebook, for iPhone. Free on the App Store.
You collect an item. You can add an annotation. Later it returns as a recollection.
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The letter
Read issue 1. Ten we kept this week, and how to keep your own. Or start with how to keep.