Unknown unknown

Unknown unknown /ʌnˈnəʊn/ (n.)
Something that you don’t know that you don’t know. This of course presents an immediate paradox, because if you know that you don’t know that you don’t know something — which for you to be talking about unknown unknowns, you must, then this is a known unknown unknown, which I rather think — but to be clear, I don’t know, as such — must be a variety of known unknown. Which makes it an unknown known unknown unknown. Since I do know that I am not sure about this I can confidently, and with some relief, rest may case at a known unknown known unknown unknown. There are six types of known.

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The Rumsfeld three:

• Known knowns: Things you do know you know;
• Known unknowns: Things you do know you don’t know;
• Unknown unknowns: Things you don’t know you don’t know;

And the Jolly Contrarian three:

• Unknown knowns: Things you don’t know you do know.
• Not officially known: Things you know but the general public can’t prove.
  
• Knowable unknowns: Things you don’t know, but think you do know, for the time being, because computers.

See also

• Risk taxonomy
• Rumsfeld’s taxonomy