When asked, "What don't you know?", what do, or should you, respond with? As there is no way to tell what you don't know (let's take for example, "x"), how can you say "I don't know X", as you've not been informed of the presence of X?

I'm also aware that the more blunt of you will state that it's normally said rhetorically or sarcastically ;)

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    When asked why we invaded Iraq yet there was no actual evidence for the weapons of mass destruction; the great American philosopher Donald Henry Rumsfeld said: "there are known knowns; there are things that we know that 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." en.wikipedia.org/wiki/There_are_known_knowns
    – user4894
    Jun 26, 2014 at 21:32
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    You don't have to go too far. When asked that question, you can simply say that you don't know, for example: whether May 17, 1934 was a tuesday or not. Then, you can denote that statement by 'S' and derive an arbitrary number of other sentences Q, Q', Q'',... that you also do not know. The trick would be to let Qs be conjunctions of S and an arbitrary true sentence. For example, if S is that 1934 sentence, you can say that you also don't know whether (S & 0 != 1) or (S & 0 != 2) or (S & 0 != k) for any k >= 3. Jun 26, 2014 at 21:48
  • Theres a fundamental distinction between knowing what you don't know and not knowing what you don't know. Jun 26, 2014 at 22:44
  • @HunanRostomyan but to say that you do not know what day of the week Sept 3 1752 was is an altogether different kind of statement. It is in fact, the only true statement you can make about it.
    – user16869
    Feb 18, 2016 at 0:20

2 Answers 2


There are a few ways in which one might try to answer such a question: some are clearly self-contradictory, but others are not. The latter kind would be the correct way to respond.

Contradictory or useless response

Asked 'what do you not know', it would be problematic to reply 'I don't know that p' for some p. But there are different reasons why it would be problematic:

  1. If you do in fact know that p is true. Then you're straightforwardly lying.
  2. Perhaps you know not-p. Then, it is plausible that you know (or at least are in a position to know) that you don't know p (since you can't believe something that is false).[^1] But this isn't very useful. Presumably, when somebody asks 'what don't you know', they mean 'what true things don't you know'.
  3. You do not know whether p or not-p. Then saying 'I don't know that p' would have the same drawbacks as (2). Since you don't know whether p is true or false, you don't know if you've answered the implied question of 'what true things don't you know'.

Knowledge wh-

The previous discussion was on answering in terms of a 'knowledge that' ascription of knowledge. But another kind of ascription of knowledge, which has has quite a bit of discussion[^2] is 'knowledge wh-' ascriptions. That is, ascriptions such as 'J.K. knows who wrote The Cuckoo's Calling', 'Isaac knows why things fall downwards', 'Rosalind knows what the structure of DNA is like' and so on.

Answering by stating your epistemic situation vis-a-vis a knowledge wh- statement would be perfectly fine:

  • I don't know who won the World Cup in 2010 (although perhaps I should!)
  • I don't know why magnets work
  • I don't know what snake tastes like
  • I don't know whether it's raining in San Francisco

The last of these gets us closest to the knowledge that statement. But in this case, we're not picking one of p or not-p to assert our lack of knowledge of. Instead, the effect is something like - whichever of p or not-p is true - that's a true statement that I don't know.

[^1]: We can give a proof of this, given some assumptions about epistemic logic. Write 'Kp' for 'I know that p'. Then, we have the proof:

  1. Kp (assumption)
  2. K¬p → ¬p (factivity of knowing - it is only possible to know what is true)
  3. Kp → p (factivity again)
  4. p (from 1 and 3 by modus ponens)
  5. ¬K¬p (from 2 and 4 by modus tollens)
  6. Kp → ¬K¬p (1 and 5, modus ponens, discharging 1)
  7. K(Kp → ¬K¬p) (Since we've proved 6 without any undischarged assumptions, we're in a position to know it. This is called the rule of necessitation)
  8. KKp → K¬K¬p (Knowledge is preserved under known entailment)
  9. Kp → K¬K¬p (Kp and KKp are equivalent. This follows from factivity in one direction, and something called the KK principle in the other. It should be noted that the KK principle is very controversial though.)

[^2]: Here are some links: Knowing the answer, Jonathan Schaffer, Questions, answers and knowledge wh-, Meghan Masto (unfortunately paywalled), Knowledge wh- bibliography on PhilPapers, T. Parent


The question "What don't you know?" seems intended to produce a state of Aporia - "An insoluble contradiction in a text's meaning; a logical impasse". It is more likely though to be a friendly invitation to a parlay: "an expression of real or pretended doubt or uncertainty especially for rhetorical effect."

I would say don't take it seriously.

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