"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, Secretary of Defense
Rene' Descartes (1596 - 1650), the French philosopher, mathematician and scientist invented the convention of representing knowns in equations by using a, b, & c (first three letters of the alphabet) and unknowns in equations with the letters x, y, & z (last three letters of the alphabet).
Essentially, Descartes' conventions describes the three forms of decisions:
Decision - making under Certainty: the "known knowns"
Decision - making under Risk: the "known unknowns"
Decision - making under Uncertainty: the "Unknown Unknowns"