Nothing is more terrible than ignorance in action.
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By the same author and on kindred topics.
He who does not know foreign languages knows nothing of his own.
Johann Wolfgang von Goethe, 1749–1832 -
Knowing is not enough; we must apply. Willing is not enough; we must do.
Johann Wolfgang von Goethe, 1749–1832 
The desire to appear clever often prevents one from becoming so.
François de La Rochefoucauld, 1613–1680 
The only safety for the vanquished is to hope for no safety.
Virgil, 70–19 BC 
They change their sky, not their soul, who rush across the sea.
Horace, 65–8 BC -
You may drive out nature with a pitchfork, yet she will always come back.
Horace, 65–8 BC