Mac has been flying for 40 years, and writing/editing aviation magazines as long as I can remember. His words here are from the September 2015 edition of EAA’s Sport Aviation magazine:
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Do something beyond
Undertake something that is difficult; it will do you good. Unless you try to do something beyond what you have already mastered, you will never grow.
~ Ronald E. Osborn, and not, as is often seen, Ralph Waldo Emerson