As David Allen, the founder of GTD, asks it: "Have you yet discovered that your mind has limited space, and it's a terrible office?" You ought to use your brain to keep things that matter to you and put the rest on paper.
Image: Stéphane Gaudry
The GTD system involves a lot of writing down what you need to do as well as organising and prioritising those tasks and projects. You don't just bother writing down what you need to get done because you'll forget it, but because it doesn't belong in your mind in the first place. If you use your mind as an office, you end up filing away tedious bits of information you don't really care about. When you fill up on useless data, the useful has nowhere to go.
Don't waste your mind as an office [GTD Times]