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).