There are ways for your brain to lie to you, which pretty much guarantee you’ll never know it. Even if someone points out the exact way in which you’re being lied to, you probably won’t accept it. Even if a being that is proven to be smarter and more right than any human being who has ever lived tells you, the chances are good you won’t believe it.

As Yudkowsky says:

I find it disturbing that the brain has such a simple macro for absolute denial that it can be invoked as a side effect of paralysis. That a single whack on the brain can both disable a left-side motor function, and disable our ability to recognize or accept the disability. Other forms of brain damage also seem to both cause insanity and disallow recognition of that insanity – for example, when people insist that their friends have been replaced by exact duplicates after damage to face-recognizing areas.

The very idea is frightening.