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.
Está lo sabido conocido: hay cosas que sabemos que sabemos. También está lo sabido desconocido, es decir, sabemos que hay cosas que no sabemos. Pero también está lo desconocido desconocido: lo que no sabemos que no sabemos.