Donald Rumsfeld, former Secretary of Defense, said this back in 2002 when responding a to a reporter’s question about links between Saddam Hussein and terrorists seeking weapons of mass destruction. It came up again Monday in a column in the Wall Street Journal. “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 not know. But there are also unknown unknowns: the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tends to be the difficult one."

I’ve always loved this quote; it’s very humbling and so true in business and life (and of course politics and war).

In short, there are things we don’t know that we don’t know we don’t know.