There are known "knowns."

Frase de Donald Rumsfeld cuando era secretario de Defensa de Estados Unidos:

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns - the ones we don't know we don't know.

"Los informes de que algo no ha sucedido siempre me interesan, porque, como se sabe, hay cosas que sabemos que sabemos. También sabemos que hay cosas que no sabemos; es decir, sabemos que desconocemos algunas cosas. Pero además hay cosas que ignoramos que no sabemos."