The essence of the Liberal outlook lies not in what opinions are held, but in how they are held: instead of being held dogmatically, they are held tentatively, and with a consciousness that new evidence may at any moment lead to their abandonment. This is the way in which opinions are held in science, as opposed to the way in which they are held in theology. The decisions of the Council of Nicaea are still authoritative, but in science fourth century opinions no longer carry any weight.

Bertrand Russell (1872-1970) English mathematician and philosopher

“Philosophy and Politics,” lecture, National Book League, London (1946-10-23)
    (Source)

Collected in Unpopular Essays (1950).