The best lack all conviction, while the worst are full of passionate intensity.
W. B. Yeats, “The Second Coming”, 1920
The fundamental cause of trouble is that in the modern world the stupid are cocksure, while the intelligent are full of doubt.
Bertrand Russell, “The Triumph of Stupidity”, 1933
The problem is that bad writers tend to have the self-confidence, while the good ones tend to have self-doubt.
Charles Bukowski, interview in Arete, 1989
(h/t to Quote Investigator)