Known Knowns

Interesting quote highlighted in a talk about responsive design by Jeremy Keith:

"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 that we know. There are known unknowns; that is to say, there are things that we now know we don't know. But there are also unknown unknowns – there are things we do not know we don't know."

- Donald Rumsfeld.