Is this actionable...NOW!
I've had a bit of an epiphany over the last week. The only way GTD makes sense is if you ask yourself, about all of your inputs, "is this actionable now?" 'Now' is the key word. I believe this is what David Allen is getting at with the question "is this actionable?" But that question is a bit misleading because, yes, there are a lot of things that it is possible to take action on. But the question is: are you going to commit to taking action now? This applies to all the 'horizons of focus'. If not actionable then they need to go to someday/maybe or to the tickler.
This may seem obvious to some. For others, it may explain why they have too many projects, goals or actions. After all, you can't do everything all at once. In essence, GTD is a system of action. If you can't take action on it, then it doesn't really belong in the system (at least in the active part of it.)