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 damn sure of what the result is going to be, he is still in some doubt. Scientific knowledge is a body of statements of varying degrees of certainty -- some most unsure, some nearly sure, but none absolutely certain.
"The Value of Science," address to the National Academy of Sciences (Autumn 1955)
