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For knowledge workers, there is a lot of work to do defining our work before we can start actually doing our work. The 5 phases of workflow is the GTD model for capturing, defining and doing our work.
Anything you keep for future reference. It is very important not to blur the lines between things that require action and those that are purely reference material.
Many mobile phones and dedicated dictation machines allow you to capture thoughts as you have them in the form of voice notes. A great collection tool to have with you at all times so you never have to remember anything again (as long as you remember to process the notes just like any other inbox).