capgras syndrome and made-up sayings

capgras syndrome (aka: delusion of doubles): fixed belief that familiar persons have been replaced by identical impostors who behave exactly like the original.

this is my favorite delusion EVER. how do you prove to these people they are delusional?
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so those of you in medicine know that one of the ways we assess psych patients is to ask them what different sayings mean like "a stitch in time saves nine" or (one that i never heard of before) "those who live in glass houses should not throw stones". the point is that people with certain psych disorders (and children) tend to answer very concretely such as "well if you stitch once then you can skip nine more stitches", whereas you would expect a normal adult to say something more abstract like "planning ahead is easier in the end".

one of my classmates was telling us this superb story today about his father: when he was a medical student he did a psych rotation at bellevue, where there were tons of homeless people wanting a clean bed and hot food who knew this game and what they should say to seem psychotic. so this doc started messing with them, using made-up sayings like
"don't hit a golden door with an iron mallet" and "when the wind blows over the bridge, the water is soon to follow".

obviously the homeless guys were baffled cause they don't mean anything but they sure sound like sayings. (if you don't believe me, say it out loud, slowly like an old wise man would).