So as a person with what I believe is a below-average ability to recognize faces (or connect them with names), I am particularly interested in Capgras delusion, which some researchers believe is the flip side of prosopagnosia, the fancy term for face blindness (a thing some people have for real, unlike me, though I have used that term to describe myself). See, the idea is that you recognize people in two ways: consciously and subconsciously. Studies have shown that people with honest-to-God prosopagnosia -- people who seriously can't tell others apart by their facial features -- nevertheless experience the same kind of subconscious neurological reaction that everyone else has to seeing people they know. So even though you don't think you recognize that guy, you kind of do. People with Capgras delusion, on the other hand, though they acknowledge that the person they think has been replaced looks just like the genuine article, don't have that subconscious response. So while your conscious mind is saying, "That's Steve's face," your unconscious mind is saying, "Something's wrong here. That's not Steve." So you come to the not unreasonable conclusion that that's an impostor posing as Steve.
But let's be honest, I'd be fascinated with Capgras delusion anyway because it's right in my wheelhouse: it's a terrifying and potentially heartbreaking example of how the human brain can go wildly wrong and still continue functioning. That's my favorite kind of thing. Just ask someone with akinetopsia (the inability to see motion, so that the world is like a series of still frames) or Riddoch syndrome (the inability to see static objects). No, those impair your ability to function, but how about something with more potentially poignant implications? How about phantom boarder syndrome, the delusional belief that someone uninvited is living in your home (usually upstairs)? Or xenomelia, the feeling that one or more of your limbs doesn't belong to you and should be removed? That's a good one. Or supernumerary phantom limb, a rare variant of phantom limb where you perceive limbs where you never had any? I'm happy to say that this can also be called reduplication of the phantom, a phrase I don't know why we're not all using constantly. Speaking of phantom limbs, I've quoted it before and I'll quote it again, chapter 108 of Moby-Dick:
Look ye, carpenter, I dare say thou callest thyself a right good workmanlike workman, eh? Well, then, will it speak thoroughly well for thy work, if, when I come to mount this leg thou makest, I shall nevertheless feel another leg in the same identical place with it; that is, carpenter, my old lost leg; the flesh and blood one, I mean. Canst thou not drive that old Adam away?
Truly, sir, I begin to understand somewhat now. Yes, I have heard something curious on that score; how that a dismasted man never entirely loses the feeling of his old spar, but it will be still pricking him at times. May I humbly ask if it be really so, sir?
It is, man. Look, put thy live leg here in the place where mine was; so, now, here is only one distinct leg to the eye, yet two to the soul. Where thou feelest tingling life; there, exactly there, there to a hair, do I. Is't a riddle?
I should humbly call it a poser, sir.
Hist, then. How dost thou know that some entire, living, thinking thing may not be invisibly and uninterpenetratingly standing precisely where thou now standest; aye, and standing there in thy spite? In thy most solitary hours, then, dost thou not fear eavesdroppers? Hold, don't speak! And if I still feel the smart of my crushed leg, though it be now so long dissolved; then, why mayst not thou, carpenter, feel the fiery pains of hell for ever, and without a body? Hah!
And that's just it, isn't it? We're not just full of phantom limbs, we're full of whole phantom other people, and sometimes they don't stay inside us where they belong. Sometimes you get a lobotomy, or a bump on the head, or you're just unlucky, and a phantom you breaks loose -- or a whole lot of them do -- and there you are, a haunted house. A whole haunted universe, with your own natural laws.