[T]he Capgras delusion [is] a bizarre affliction that occasionally strikes human beings who have suffered brain damage. The defining mark of the Capg… - Daniel Dennett

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[T]he Capgras delusion [is] a bizarre affliction that occasionally strikes human beings who have suffered brain damage. The defining mark of the Capgras delusion is the sufferer's conviction that a close acquaintance (usually a loved one) has been replaced by an impostor who looks like (and sounds like, and acts like) the genuine companion, who has mysteriously disappeared! … What is particularly surprising about these cases is that they don't depend on subtle disguises and fleeting glimpses. On the contrary, the delusion persists even when the target individual is closely scrutinized by the [Capgras sufferer], and is even pleading for recognition. Capgras sufferers have been known to murder their spouses, so sure are they that these look-alike interlopers are trying to step into their shoes — into whole lives — that are not rightfully theirs! There can be no doubt that in such a sad case, the [sufferer] in question has deemed true some very specific proposition of nonidentity: This man is not my husband; this man is a qualitatively similar to my husband as ever can be, and yet he is not my husband. Of particular interest to us is the fact that people suffering from such a delusion can be quite unable to say why they are so sure.