"There are no knowns. There are things we know that we know. There are known unknowns - that is to say, there are things we now know we don't know but there are also unknown unknowns.
There are things we do not know we don't know. So when we do the best we can and we pull all this information together, and we then say well that's basically what we see as the situation, that is really only the known knowns and the known unknowns. And each year we discocover a few more of those unknown unknowns."
(from an unknown)