Ignorance more frequently begets confidence than does knowledge

Three great quotes from Wikipedia's article on the Dunning-Kruger Effect.

From Charles Darwin in The Descent of Man

Ignorance more frequently begets confidence than does knowledge

From Bertrand Russell

One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision

and from William Shakespeare in As You Like It

The fool doth think he is wise, but the wise man knows himself to be a fool