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 made the quote as a kind of excuse for the U.S. army in Iraq which still haven’t found the WMD used as the pretext for the Iraq War. Basically, Rumsfeld was making a diversion because he’s unable to respond to the allegations that no WMD existed then in Iraq. But curiously, aside from the context, the statement itself is a discussion of knowledge.
As Donald Rumsfeld said it, there are three kinds of things: known knowns, known unknowns, and unknown unknowns. But usually we only talk about the first two.
- Known knowns. Things that we know we know. E.g.: everything in the universe are made up of atoms.
- Known unknowns. Things that we know we don’t know. E.g.: God exists/don’t exists.
The unknown unknown is clearly difficult to talk about, because how can we talk about something that we don’t even know what it is? For example, imagine that you were preparing for school at night. You haven’t forgotten your physics and math textbooks. You have already had your homework done and put it to your backpack. But the next day, you had a chemistry exam and of course you didn’t study for it because you don’t even know that there will be a chemistry exam. This is what an unknown unknown is like.
Slavoj Zizek (sorry I didn’t write your name with your fancy accents), a Slovenian philosopher, adds a fourth category: the unknown knowns. It can mean two things: a) something we have knew, but have forgotten or b) something that we know, but unaware of knowing.