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When you do not know a fact you are ignorant. My question is what would you call the state of believing you know something that is false.

For instance suppose I see a black box and having opened it earlier, I know there is a rabbit in it. Some time later unbeknownst to me someone removes the rabbit. I still 'know' there is a rabbit in the box.

What do you call this state?

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There's always the Dunning-Kruger effect. – Hot Licks Aug 15 '15 at 15:50
    
Denial comes to mind. – moonstar2001 Jun 10 at 14:23
up vote 7 down vote accepted

One word is mistaken.

adjective
  wrong in one’s opinion or judgement:
    she wondered whether she’d been mistaken about his intentions
      based on or resulting from a misunderstanding or faulty judgement:
        don’t buy a hard bed in the mistaken belief that it is good for you
        an unfortunate case of mistaken identity

[ODO]

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And, pace @Fortiter, a mistaken belief is easily proved to be wrong. – St John of the Cross Mar 27 '13 at 8:04

Such a belief could be described as erroneous. Merriam-Webster defines erroneous as "containing or characterized by error: mistaken."

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I think that this example is merely a matter of ignorance: the person who believes that there is a rabbit in the box is ignorant of the fact that the rabbit was removed and, therefore, continues to believe something that once was true but has since changed and become false.

A better word than ignorant and mistaken, I think, is one of these:

tricked, bamboozled, conned, deceived, duped, misled

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All those words require deliberate action on the part of a third party, and while the OP's example does include that, it's not necessarily a required feature of the requested term. – user867 Jun 5 '13 at 4:18

You believe that the rabbit is in the black box.

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Do I know that the sun will rise tomorrow?

I think that all I know is that there is no recorded instance in history of the sun having failed to rise on any morning.

So it would be stretching things for anyone to claim that because there was a rabbit in the box on Monday, that they knew there to be a rabbit in the box today - Thursday.

So I call the state delusion.

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Exactly my point! I tried to make that point regarding my answer of in denial but some people don't seem agree. But whatever, I am glad to see that someone else here sees value in giving answers from different perspectives. – Sk Johnson Oct 25 '15 at 13:52

I would say they have the wrong impression or that they are under the impression that the rabbit is in the box. But you wanted a single word so why not just say the person is uninformed, or unaware?

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I don't understand why some answers without any sources cited or definitions offered earn up votes, or at least stay at 0, while other answers on the same thread will be voted down for no legitimate reason. Whatever... I fixed it, can someone please fix my negative score now? Or tell me how my answer is 'wrong', if not? – Sk Johnson Oct 25 '15 at 11:50
    
I didn't downvote, but I suspect your answer was downvoted because you didn't read the question carefully, so you answered a year-old question with a completely irrelevant answer. If you mistakenly think there is beer in the refrigerator because you bought some, but unknown to you somebody else drank it, you are not "in denial". – Peter Shor Oct 25 '15 at 12:11
    
I don't think it is irrelevant, the person should have looked in the box again to verify what they believed was true. Avoiding being confronted with the truth is a form of denial so how is it irrelevant? Also, If the question is so old why is it appearing on the first page of recent questions results? Something wrong with the software or configuration if that is of concern. – Sk Johnson Oct 25 '15 at 12:21
    
If you think in denial is a good answer to the original question, I don't think you understand what in denial means in English. – Peter Shor Oct 25 '15 at 12:22
    
I understand it perfectly well. What is the difference between that and cognitive dissonance, which got voted up by the way. – Sk Johnson Oct 25 '15 at 12:24

Perhaps cognitive dissonance if you are aware of the contradiction, or doublethink if you are unaware of any problem.

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I consider this "the state of false perception"

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Hello, Sam, and welcome to English Language & Usage. Your view appears to be in line with Plato's—or rather, with the character Socrates'—in his dialogue Meno, which attempts to distinguish 'truth' from 'true opinion' and 'false opinion.' I will gladly upvote your answer if you link it to some plausible discussion or argument in an authoritative source, but at the moment it reads simply as a personal opinion. – Sven Yargs May 12 '15 at 18:52
    
Ditto this comment to Mark? – ab2 Aug 15 '15 at 21:46

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