A woman looks out of a window at the Big Ben clock tower in London, Britain, April 26, 2017. REUTERS/Hannah McKay

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You can't be the best at everything.Thomson Reuters

Time — at least when it comes to an individual person's life — is a finite resource. Spend it on one thing and you won't be able to spend it another.

On a micro level, this is pretty obvious: If you spend the entire morning answering emails, you definitely won't have that project report ready by noon.

On a macro level, this logic is harder to internalize: You probably can't be a top-notch mom, manager, community board member, yogi, and musician. At least not all the time. (And if you've found that to be false, please share with us your secrets.)

In his new book, "Barking Up the Wrong Tree," Eric Barker — who runs a popular blog by the same name as the book — shares a story that can help readers come to terms with those temporal limitations.

Barker shares the story of Spencer Glendon, a partner at a big money-management firm who was a Fulbright Scholar and got his Ph.D. in economics from Harvard.

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He's also spent much of his life battling serious illness: In high school, Barker writes, Glendon suffered from chronic ulcerative colitis. When he got an organ transplant, he had to go through immunosuppressive therapy, leaving him with a weakened immune system.

In high school, when he was seriously ill, Glendon saw a therapist who suggested that he focus on accomplishing one thing a day — even if that thing was simply making dinner.

Barker writes:

"Coming to terms with his illness taught him something thatalmost all of us overlook: Everything we do in life is atrade-off. Choosing to do one thing means not doing somethingelse. There was no way for Spencer to say 'I want to do this'without also saying 'And I'm willing to give that upto do it.'"

Barker recommends:  

"Imagine you were Spencer at his lowest point. Whatwould you do if you were ill and could manage only one task perday? Congratulations. You now know what matters to youlost, what should get the most hours, what should be donefirst. You know where you should apply grit, and by the sametoken, what you should quit."

Everyone has limitations — in Glendon's case, it's his physical abilities, but in someone else's case, it might be energy, attention, or time. A big key to success is working within those limitations and being the best you can be at something, not everything.