If you're a fan of Getting Things Done, you're familiar with the Four Criteria Model for choosing tasks. It's where the rubber meets the road in GTD, because it's the way you decide, in the moment, how any one of those wonderful tasks you've been tracking in your big system actually gets done.

As common sense as it seems to GTD'ers, this model is one of the more controversial aspects of Getting Things Done for a simple reason: it posits that priority is not the only factor in deciding what to do at a given time. It's just one of four factors, which include, all told:

1. Context - Where are you? What tools are available? What are the limits and possibilities unique to this moment?
2. Time available - Do you have, for example, 30 seconds, 30 minutes, or 30 hours available to you right now? What tasks could you accomplish given the time you have?
3. Energy available - Are you full of energy, is your ass dragging, or are you somewhere in between? Which of the tasks on your list could you finish, given that energy level?
4. Priority - If you had access to all the tools, opportunities, time, and energy you needed, what's the most important or time-sensitive thing you could do right now?

When I'm helping coach people on getting it together, they're often puzzled by this seeming bit of new-agery -- partly, I suspect, because most of us have been conditioned all our lives to think that pre-ordained Priority stamps always trump everything, all the time, always, forever, in all cases, end of story. But is it true, reasonable, or even physically possible to always work this way? Can you will yourself into doing only your identified high-priority items anytime, all the time?

Nope, and I'll show you one reason why.