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.
——Donald Rumsfeld
有已知的已知,即我们知道有些事情我们知道。我们也知道有已知的未知;也就是说,我们知道有些事情我们不知道。但也有未知的未知数,即我们不知道的未知数。