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.

"Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do no know. But there are also unknown unknowns -- the ones we don't know we don't know

There's always the risk that there are unknown unknowns.

Don't do or say things you would not like to see on the front page of The Washington Post.

You have to be aware that there are some unknown unknowns out there, ... a few well-defined robotic missions (to the moon) will give us most of the answers that we need.

Our task, your task... is to try to connect the dots before something happens. People say, 'Well, where's the smoking gun?' Well, we don't want to see a smoking gun from a weapon of mass destruction.