When Donald Rumsfeld was Secretary of Defense, he coined some phrases about knowledge that apply far beyond military matters.
Secretary Rumsfeld pointed out that there are some things that we know that we know. He called those "known knowns." We may, for example, know how many aircraft carriers some other country has. We may also know that they have troops and tanks, without knowing how many. In Rumsfeld's phrase, that would be an "unknown known" — a gap in our knowledge that we at least know exists.
Finally, there are things we don't even know exist, much less anything about them. These are "unknown unknowns" — and they are the most dangerous. We had no clue, for example, when dawn broke on September 11, 2001, that somebody was going to fly two commercial airliners into the World Trade Center that day.
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Thomas Sowell (photo)