... overconfidence in one's ...

... overconfidence in one's own ability is the root of much evil. Vanity, egoism, is the deadliest of all characteristics. This vanity, combined with extreme ignorance of conditions the knowledge of which is the very A B C of business and of life, produces more shipwrecks and heartaches than any other part of our mental make-up.
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 Alice Foote MacDougall

Quotes from the same author

You realize the futility of worry. You learn to hate the small and the little. Life is a pie which you cut in large slices, not grudgingly, not sparingly. You know your limitations and proceed to eliminate them; your abilities, and proceed to develop them. You are free.
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 Alice Foote MacDougall
When one is working out a problem ... life becomes duality. One's ego transacts the ordinary routine of things, as if the mind had an upper and lower story and the regular performance of the day's duties moved and motivated on the upper floor, while down below the all-absorbing problem toils silently, forcefully, toward its solution.
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 Alice Foote MacDougall
Success is an absurd, erratic thing. She arrives when one least expects her and after she has come may depart again almost because of a whim.
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 Alice Foote MacDougall
You the rich are no whit more attractive or capable than you who were poor and struggling a few years back. But when before you plodded lonely and unappreciated, now the glamour of the motor and the smart apartment surrounds you with a tangible glory. It is amazing how many friends look you up, call you by name, and extol you, who were once a little timid, or indifferent, or utterly neglectful in your time of dire poverty. One has true friends when one is poor and no riches can be greater than that. They are not so obvious when one is rich.
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 Alice Foote MacDougall
Imagination is a valuable asset in business and she has a sister, Understanding, who also serves. Together they make a splendid team and business problems dissolve and the impossible is accomplished by their ministrations.... Imagination concerning the world's wants and the individual's needs should be the Alpha and Omega of self-education.
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 Alice Foote MacDougall