When Donald Rumsfeld came out with his line about there being “unknown unknowns”, a lot of people laughed, and in response his defenders sneered at the laughers. But I didn’t see on either side a real appreciation of the phrase — indeed, I still haven’t, from anyone.
These are unknowns.
They are not the “known unknowns”, which “we know we don’t know”.
Instead they are things which “we don’t know we don’t know”.
So these were things he (and others) thought he knew, but he didn’t know — in simpler words, things he was wrong about.
This makes Rumsfeld’s line one of the most unusual things said by a politician in recent memory: an admission of error. Not just that he had been wrong in the past — as in the line, which politicians hate, but are sometimes forced into, “yes, that was a mistake, but now I know better”. This, though presented confusingly, was an even rarer admission: that he was wrong in the present and going to be wrong in the future. If he’d wanted not to obfuscate but to put it dramatically, he could have turned one of Shakespeare’s lines against himself, saying “There are more things in heaven and earth than are dreamt of in my philosophy”.
Which indeed turned out to be the case.