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A commonplace journal is defined as a form of knowledge keeping, typically by compiling information in notebooks and whatnot. Any sort of thing can go in a commonplace journal. Things that are most commonly found are quotes, adages, notes... whatever sort of things that one would want to reference later.

I carry around notebooks and sketchbooks with me regularly as a way to catalog ideas. Here's a collection of quotes I've amased over time, both for personal reference and to share with others.