“Stupidity is the same as evil if you judge by the results.” ― Margaret Atwood, Surfacing

“Talk sense to a fool and he calls you foolish.” 

― Euripides

“Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.” 

― Albert Einstein

“You have attributed conditions to villainy that simply result from stupidity.” 

― Robert A. Heinlein, The Green Hills of Earth

“Talk, talk, talk: the utter and heartbreaking stupidity of words.” 

― William Faulkner, Mosquitoes