This is a terrific quotation from Richard Feynman on the nature of Scientific Knowledge, and specifically the necessity for skepticism. My paraphrase: There is always room for doubt. If the humanities would like to take something from the sciences, as it seems to want to do sometimes these days given their relative funding and cultural prestige, they ought to begin here:

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.

For more on this and others of Feynman's ideas concerning scientific responsibility, see Maria Popova's discussion at Brain Pickings.