A CEO client was struggling with more than 900 emails in his inbox waiting for his attention. That may not seem like many if you’re sitting there with 15,000 messages in your inbox—most of them worthless “reply all” garbage or Pottery Barn promotions—but all of these were significant, important, and potentially valuable emails that he didn’t want to delete.

He was trapped in a common fallacy: that value is fixed and unchanging.

It’s not. In a world of constraints, importance and value is contextual.

If you were Warren Buffett—if you had no financial constraints—you’d never have to decide what to spend your money on, because you could buy pretty much anything and everything you wanted. Send your kid to a private college. And buy a Rolls Royce. And a $35,000 watch. And take the family on a luxury safari in Botswana. But of course, you probably don’t have as much money as Warren Buffett (you wouldn’t be reading my blog if you did), so every purchase you make presents an opportunity cost: pay for college, and wear a Casio while driving your Geo Metro.

The same equation holds for time. Your time and attention—like your money—is finite. If it were infinitely elastic, you could go to all your meetings, answer all your emails, visit all your customers, and coach all your employees. But of course, it’s not—so you can’t. There’s an opportunity cost every time you choose to do one thing, because it precludes you from doing something else.

Importance and value also degrade over time. Today’s opportunity is often less valuable in three months or three years. Being the first to market brings enormous rewards that competitors struggle to capture. Thanking an employee today for a job well done is more important than recognizing her in a month. Information about your market is important today, but not so much next quarter. As the old saying goes, today’s newspaper is tomorrow’s fish wrapper.

Those 900 emails in the CEO’s inbox? As each one came in, he determined that it was “important” and saved it so that he could come back to it. But the value of each email changed in the context of all 900. Many of them, while important when they arrived, were just not all that important four months and 700 emails later. He could have easily deleted them, but he was stuck in the mindset that they still retained value.

This situation reminds me of something that Jim Lancaster, president of Lantech, wrote in his terrific new book The Work of Management. He explained how his team was spending all its time cataloging errors and managing a database of different kinds of production defects instead of actually fixing the problems when they occur. Not surprisingly, they didn't make much progress on finding and fixing root cause. The big shift came when they focused on taking care of current problems, without worrying about their relative importance. As he writes, “Solving the problem that presents itself now is more valuable than attacking the most important problem we have.”

To be fair, it’s not as though those emails are going to sink his company or prevent him from doing a great job. But if you believe in 5S for the shop floor or the office, then you should apply that tool to those no-longer-valuable emails. Holding onto them is the equivalent of holding onto the junk that’s piled up in the corner of your warehouse, or the tools and jigs for products that you no longer make.

More importantly, there’s a real psychic burden that comes from those messages. The CEO was weighed down by the feeling that he should be doing something about those emails, and felt guilty that he wasn’t on top of them. That pointless psychic burden was a distraction, and wasn’t helping him do his job.

Personal lean point #39: Make friends with your delete key. Recognize the contextual nature of value so that you can let go of the old stuff and focus on what really is important.