With Donald Rumsfeld’s book Known and Unknown making the rounds, I keep hearing the speech that gave the book its title:
[T]here 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.
This idea is not new. I don’t know who was the first ever to use this concept, but an old usage and favorite of mine comes from the best book every written on investing, Reminiscences of a Stock Operator:
Among the hazards of speculation the happening of the unexpected–I might even say of the unexpectable–ranks high.