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

I've just always been a bit of a dork.

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.

There's a lot of unknowns going in, but I think every player, until he plays, has a degree of unknown about him. I think he's earned this opportunity. If indeed Jerome can't go, I'm not concerned. I'm rather excited.

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.

Basically, books were a luxury item before the printing press.