

Variant: The problem with the world is that the intelligent people are full of doubts, while the stupid ones are full of confidence.
Often paraphrased as "The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts."
Compare: "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." B. Russell, New Hopes for a Changing World (1951). Compare also: "The best lack all conviction, while the worst / Are full of passionate intensity." W. B. Yeats, The Second Coming (1919).
See also: Dunning-Kruger effect, Historical Antecedents https://en.wikipedia.org/wiki/Dunning–Kruger_effect#Historical_antecedents.
1930s, Mortals and Others (1931-35)
Variant: The problem with the world is that the intelligent people are full of doubts, while the stupid ones are full of confidence.
“It was the short men that caused all the trouble in the world.”
Source: Goldfinger
“There is no force more potent in the modern world than stupidity fueled by greed.”
Source: A Voice Crying in the Wilderness (Vox Clamantis in Deserto) (1990), Ch. 11 : Money Et Cetera, p. 100
“The mystic too full of God to speak intelligibly to the world.”
Arthur Rimbaud.
The Symbolist Movement in Literature (1899)
Sam, Sam, Pick Oop Tha' Musket
“One could argue that most of the trouble in the world is caused by introspection.”
Source: A Long Way Down