Not to be absolutely certain is, I think, one of the essential things in rationality.

Bertrand Russell (1872-1970) English mathematician and philosopher

“Am I an Atheist or an Agnostic?” sec. “Don’t Be Too Certain!” (1949)
    (Source)

Originally given as a speech, "Agnosticism v. Atheism," Rationalist Press Assoc. Annual Dinner, London (1949-05-20), then printed as "Agnosticism v. Atheism," The Literary Guide and Rationalist Review (1949-07), then released as an essay under this title later in 1949.