a mathematical metaphor.

The key insight of both Peters and Collins is that we spend too much time on addition and not nearly enough on subtraction.

~ from Think Tank: Why we all need a 'To Don't' List: Two management leaders reveal a simple but powerful idea for achieving high performance – deciding what not to do.

the article author puts forward his own don't-do list. among them:

• Don't answer email during peak morning writing hours;
• Don't drink coffee in the afternoon;
• Don't go to sleep after 11pm.

i agree with the first "don't" -- the pristine time of the morning, when your brain is clear and you can entertain ideas with a fresh perspective, is too valuable to waste on email.

on the other hand, it seems silly to restrict deliberately one's workload in the later hours of the day ..