There are six types of known.

The Rumsfeld three:

And the Jolly Contrarian three:

In which the curmudgeonly old sod puts the world to rights.
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Knowable unknown
/ˈnəʊəbᵊl ʌnˈnəʊn/ (also “constructive known”) (n.)
Something you don’t know, and can’t know, but you believe you can as good as know as long as you have enough data and clever enough algorithms.

Such as that options will behave according to the Black-Scholes option pricing model.

the problem with knowable unknowns is that they are irritatingly time-bound, and tend to be knowable at all the times where you don’t really need to know them, but then become suddenly impenetrably unknowable just when your pants are down.

This is not from Donald Rumsfeld’s taxonomy, but the JC’s.

See also