Paraphrasing and expanding on Rumsfeld…
Known knowns - things we know we know;
Known unknowns – to say we know there are some things we do not know;
Unknown unknowns - the ones we don't know we don't know;
Unknown knowns – stuff you know, but don’t know that you know it.
And if one looks throughout the history of our country and other free countries, it is the latter two categories that tend to be the difficult ones.
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