Unknown unknowns...

I've always thought that Donald Rumsfeld, whatever other defects he may have had (I am not, generally speaking, among his defenders), got a bad rap for saying:

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 not know. But there are also unknown unknowns — there are things we do not know we don't know.

This is a nice and succinct summary of knowledge...