Unknown unknown

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Unknown unknown /ʌnˈnəʊn/ (n.)
Something that you don’t know that you don’t know, as famously articulated by Donald Rumsfeld. 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 my case at a known unknown known unknown unknown. There are five types of known.

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