know when to quit
As we now say, "Know that No.. [Spending time on things that don't concern you is worse than doing nothing. [Be careful not only to stay out of the way of others, but to stay out of the way of others. [You don't have to serve others so much that you lose time for yourself. [Don't abuse the help of friends.
The second half of the story, around "choose the best of the best", was also discussed in Excellence resides in quality, not quantity. Don't be a flunky.
from The Art of Worldly Wisdom
Know when to pull back.
xxxiii
If knowing how to deny is a great lesson in life, knowing how to deny oneself with respect to both matters and persons is an even greater lesson. There is extra work that eats up valuable time. Being preoccupied with things that don't concern you is worse than doing nothing. It is not enough for an attentive person to stay out of the way of others; he must be careful that others stay out of his way. One need not belong to all so much as not belong to oneself at all. Even to friends one must not abuse their help or demand more than they themselves will allow. Every excess is a failure, especially in personal intercourse. Wise moderation in this respect is the best way to maintain the goodwill and respect of all. Thus you will be selective in your talents and never sin against the unwritten rules of good taste.
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