The scientist has a lot of experience with ignorance and doubt and uncertainty, and this experience is of very great importance, I think. When a scientist doesn’t know the answer to a problem, he is ignorant. When he has a hunch as to what the result is, he is uncertain. And when he is pretty darn sure of what the result is going to be, he is in some doubt. We have found it of paramount importance that in order to progress we must recognize the ignorance and leave room for doubt. Scientific knowledge is a body of statements of varying degrees of certainty — some most unsure, some nearly sure, none absolutely certain.

Richard Feynman (1918–1988) American physicist

Speech (1955-11), “The Value of Science,” National Academy of Sciences Autumn Meeting, California Institute of Technology
    (Source)

Collected in (1955-12), "The Value of Science," Engineering and Science, Vol. 19.