Brute force plays a much larger part in the government of the world than it did before 1914, and what is especially alarming, force tends increasingly to fall into the hands of those who are enemies of civilization. The danger is profound and terrible; it cannot be waved aside with easy optimism. The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.

Bertrand Russell (1872-1970) English mathematician and philosopher

“The Triumph of Stupidity,” New York American (1933-05-10)
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Often paraphrased, "The trouble with the world is that the stupid are cocksure, and the intelligent are full of doubt." See also Yeats and Bukowski.

More examination of this quotation: The Best Lack All Conviction While the Worst Are Full of Passionate Intensity – Quote Investigator.