From Science-Based Medicine:
Neurologist Robert A. Burton, MD has written a gem of a book: On Being Certain: Believing You Are Right Even When You’re Not. His thesis is that “Certainty and similar states of ‘knowing what we know’ arise out of involuntary brain mechanisms that, like love or anger, function independently of reason.” Your certainty that you are right has nothing to do with how right you are.
Within 24 hours of the Challenger explosion, psychologist Ulric Neisser had 106 students write down how they’d heard about the disaster, where they were, what they were doing at the time, etc. Two and a half years later he asked them the same questions. 25% gave strikingly different accounts, more than half were significantly different, and only 10% had all the details correct. Even after re-reading their original accounts, most of them were confident that their false memories were true. One student commented, “That’s my handwriting, but that’s not what happened.”
Just as we may “know” things that clearly aren’t true, we may think we don’t know when we really do. In the phenomenon of blindsight, patients with a damaged visual cortex have no awareness of vision, but can reliably point to where a light flashes when they think they are just guessing. And there are states of “knowing” that don’t correspond to any specific knowledge: mystical or religious experiences.