best lack all conviction

The best lack all conviction, while the worst
Are full of passionate intensity.

-- The Second Coming, 1920, W. B. Yeats

The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.

-- “The Triumph of Stupidity”, 1933 by Bertrand Russell

But the problem is that bad writers tend to have the self-confidence, while the good ones tend to have self-doubt.

-- from 1989 interview with Charles Bukowski

The problem with the world is that the intelligent people are full of doubts while the stupid ones are full of confidence

-- @Bukowskiquot