In 2002, after a Pentagon news briefing, the then US secretary of defence Donald Rumsfeld was widely ridiculed for his thoughts about knowledge. Discussing the issue of whether Iraq was supplying weapons of mass destruction to terrorists, he said:
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.
