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)
Bertrand Russell: Trending quotes
Bertrand Russell trending quotes. Read the latest quotes in collection“Do not fear to be eccentric in opinion, for every opinion now accepted was once eccentric.”
"A Liberal Decalogue" http://www.panarchy.org/russell/decalogue.1951.html, from "The Best Answer to Fanaticism: Liberalism", New York Times Magazine (16/December/1951); later printed in The Autobiography of Bertrand Russell (1969), vol. 3: 1944-1967, pp. 71-2
1950s
Context: The Ten Commandments that, as a teacher, I should wish to promulgate, might be set forth as follows:
1. Do not feel absolutely certain of anything.
2. Do not think it worth while to proceed by concealing evidence, for the evidence is sure to come to light.
3. Never try to discourage thinking for you are sure to succeed.
4. When you meet with opposition, even if it should be from your husband or your children, endeavour to overcome it by argument and not by authority, for a victory dependent upon authority is unreal and illusory.
5. Have no respect for the authority of others, for there are always contrary authorities to be found.
6. Do not use power to suppress opinions you think pernicious, for if you do the opinions will suppress you.
7. Do not fear to be eccentric in opinion, for every opinion now accepted was once eccentric.
8. Find more pleasure in intelligent dissent that in passive agreement, for, if you value intelligence as you should, the former implies a deeper agreement than the latter.
9. Be scrupulously truthful, even if the truth is inconvenient, for it is more inconvenient when you try to conceal it.
10. Do not feel envious of the happiness of those who live in a fool's paradise, for only a fool will think that it is happiness.
Source: 1940s, A History of Western Philosophy (1945)
Variant: The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts.
It is true that this proviso is hardly necessary as regards the multiplication table, but knowledge in practical affairs has not the certainty or the precision of arithmetic. Suppose I say "democracy is a good thing": I must admit, first, that I am less sure of this than I am that two and two are four, and secondly, that "democracy" is a somewhat vague term which I cannot define precisely. We ought to say, therefore: "I am fairly certain that it is a good thing if a government has something of the characteristics that are common to the British and American Constitutions," or something of this sort. And one of the aims of education ought to be to make such a statement more effective from a platform than the usual type of political slogan.
1940s, Philosophy for Laymen (1946)
Source: Introduction to 1961 edition of Sceptical Essays (1961)
"The Moral Problem"
1920s, Why I Am Not a Christian (1927)
"The Emotional Factor"
1920s, Why I Am Not a Christian (1927)
1940s, The Bomb and Civilization http://personal.kent.edu/~rmuhamma/Philosophy/RBwritings/bombCivilization.htm (1945)
What is a Christian? https://users.drew.edu/~jlenz/whynot.html (1927)
1920s, Why I Am Not a Christian (1927)
“It seems that sin is geographical.”
From this conclusion, it is only a small step to the further conclusion that the notion of "sin" is illusory, and that the cruelty habitually practised in punishing it is unnecessary.
A Fresh Look at Empiricism: 1927-42 (1996), p. 283
Attributed from posthumous publications
1900s, "The Study of Mathematics" (November 1907)
Source: 1910s, Our Knowledge of the External World (1914), p. 21
"Fear, the Foundation of Religion"
1920s, Why I Am Not a Christian (1927)
Source: 1930s, Education and the Social Order (1932), p. 110