“If you want to leave your footprints on the sands of time, be sure you’re wearing work shoes.”

“The glamour's off. Almost any question you ask can be answered. It's only the questions that you didn't know to ask that remain, dancing the can-can behind your back. The unknown unknowns.”

“There are known knowns, things we know that we know; and there are known unknowns, things that we know we don't know. But there are also unknown unknowns, things we do not know we don't know.”

“You do not travel if you are afraid of the unknown, you travel for the unknown, that reveals you with yourself.”

“We shall not fear the unknown. We live in the unknown... make the unknown known today as tomorrow is yet to come. Never fear it. Be grateful to see it".”

“Only the unknown frightens men. But once a man has faced the unknown, that terror becomes the known.”

“Artists live in unknown spaces and give themselves over to following something unknown.”

“For any scientist the real challenge is not to stay within the secure garden of the known but to venture out into the wilds of the unknown.”

“You laugh because what's fearful and unknown is also what's funny, you laugh the way a small child will sometimes laugh and cry at the same time when a capering circus clown approaches, knowing it is supposed to be funny... but it is also unknown, full of the unknown's eternal power.”

“[Rumsfield's] reply included a complex formulation that would become inextricably associated with him: 'There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.”

“Design is an unknown.”