You've really done your job when what's true? (Challenging question for all of us!) - David Allen

When you know how, any time, to get to the place where it's all sort of funny, you've mastered GTD.

As to methods there may be a million and then some, but principles are few. The man who grasps principles can successfully select his own methods. The man who tries methods, ignoring principles, is sure to have trouble. — Ralph Waldo Emerson Used

If you figured out why making a list reduces overwhelm & confusion, you'd keep nothing in your head the rest of your life.