What is the correct usage of phrase "you don't know what you don't know"? Can it be used in formal conversation/writing?
When expressing frustration or exasperation, we often begin by saying something like, "If I had only known..."
- If I had known your plane was going to be two hours late, I wouldn't have rushed to the airport.
- If I had known you were going to get such bad grades on your report card, I wouldn't have let you go to the dance last weekend.
- Had I known the van was going to overheat, I would have driven the car instead.
Yet, in all these instances, life offers no crystal ball; there's no way of knowing what the future holds, or to be cognizant of something we are unaware of. This is the essence of "You don't know what you don't know," only it's being expressed in a comical way, much like Yogi Berra might have said.
(To those unfamiliar with the Yogi Berra reference, he was a professional baseball player who was reknowned for his wry way of saying things, such as, "It ain't over 'til it's over," and, "You can observe a lot by watching." That said, you couldn't know what you didn't know, but now you know.)
Generally speaking, it would not be a good way to express something in formal writing, unless perhaps you were deliberately injecting humor.
Socrates said 'I know that I know nothing', which seems to cover the sentiment. I'd say his version is worthy of formal writing or conversation.
One of several sources of this and similar phrases is a speech by US Secretary of Defense Donald Rumsfeld on February 12, 2002:
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. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.
Whether "unknown unknowns" can be considered "formal" or not in the strictest sense, it is, thanks to this speech, a familiar phrase, reasonably well-understood, to folks in the US who are older than maybe 30 years of age.
I wouldn't necessarily go as far as call this a "phrase" per se. For me it's a confusing sentence; ambiguous at least.
Whether you can use it or not depends solely on who your audience is. If you're sure the person you're talking to is bound to understand what you're trying to say, then I don't see any problem with using the phrase.
However, if you're unsure the people you're talking to might get confused by the sentence, then definitely don't use it.
Why would you say such a thing anyway? Are you trying to look professional or intelligent? Then use statements which are neither ambiguous, nor difficult to understand in any other way.
It has two very different meanings, that you would have to distinguish by context.
One meaning is just expressing that we have limitations. If you don't know something, that's just tough, you don't know it and you have to live with that. You don't have information if you don't have that information.
The other meaning is that not only are there gaps in our knowledge, but often we don't even know what the gaps in our knowledge are. I don't know how to speak Finnish. That's a gap in my knowledge that I know about. I know that I don't know how to speak Finnish. But there are gaps in my knowledge that I'm not even aware of. That's where you can say "You don't know what you don't know" meaning that you don't even know what knowledge you are missing.
I haven't heard or read this expression often enough. But the meaning of 'you don't know what you don't know' is clear to me without being ambiguous. However, I feel it may sound a bit rude to some people and so, I wouldn't want to use it in formal conversations or writing unless I know well the person I am saying it to.
This is similar to "you know what you don't know".
I think this expression is used when someone is making statements as if fact when they are just opposite. For example, someone speaking about Darwinism old school as a fact. The one speaking knows about something about theory without proof. This is "you know what you don't know".
It was explained to me along these lines. Each of us has different (spheres of) knowledge, however:
- we all know that we know some things (whatever they may be, e.g. "if I tie my shoelaces this way they usually stay done up");
- we all know that we don't know some other things (whatever they may be, e.g. for some of us what are the best brain surgery procedures); and lastly,
- there are things that each of us don't know that we don't know (I take this to mean they are completely off each of our radars) — and whatever is in this 'sphere' differs from person to person (along with the other two).