I think nobody should be certain of anything. If you’re certain, you’re certainly wrong, because nothing deserves certainty, and so one ought always to hold all one’s beliefs with a certain element of doubt and one ought to be able to act vigorously in spite of the doubt.

Bertrand Russell (1872–1970) English mathematician and philosopher

Interview by Woodrow Wyatt, BBC TV (1959)
    (Source)

Collected in Bertrand Russell's BBC Interviews (1959) [UK] and Bertrand Russell Speaks His Mind (1960) [US]. Reprinted (abridged) in The Humanist (1982-11/12), and in Russell Society News, #37 (1983-02).