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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.
The physical, emotional or intellectual horsepower you need to complete an action. It's good to always have some really easy things you can do when you're not feeling on top of your game!
The practice of keeping strict boundaries between items of different types. In GTD, for example, it is highly recommended to keep reference material (which has no emotional cost) separate from commitments and actions you need to keep (which have strong emotional pull). If you blur the edges between items of different types, your system will not be effective.