Unknown unknown

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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 we rather think — but to be clear, we don’t know, as such — must be a variety of known unknown. Which makes it an unknown known unknown unknown. I am pretty certain about this. That is 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