Donald Rumsfeld said this quote

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.

~ Donald Rumsfeld

Those who made the decisions with imperfect knowledge will be judged in hindsight by those with considerably more information at their disposal and time for reflection.

~ Donald Rumsfeld

There are known knowns, things we know that we know; and there are known unknowns, things that we know we don't know. But there are also unknown unknowns, things we do not know we don't know.

~ Donald Rumsfeld

If in doubt, don't. If still in doubt, do what's right.

~ Donald Rumsfeld