It's an idea that recurs throughout history. We've got Socrates and Shakespeare already mentioned in the thread, to which I'll add:
W B Yeats: "The best lack all conviction, and the worst / Are full of passionate intensity."
Bertrand Russell: "The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt."
Charles Darwin: "Ignorance more frequently begets confidence than does knowledge."
the Tao Te Ching: "To know that you do not know is highest. To not know but think you know is flawed. ... The sages are without fault, because they recognize the fault as a fault".
(Semi-interestingly, I can't find anything very close to this idea in the biblical books of Proverbs and Ecclesiastes -- the former sees certainty as something to aspire to, and the latter is just equally down on everyone, wise and foolish alike.)