As I was washing my hands last night, the angle of the light combined with the shadow cast by the raised tendon beneath the skin of my flexed left hand to create the illusion that I had a bloody gash in my arm. In almost the same instant I noticed the gash, I realized it was an illusion, but during some number of nanoseconds it seemed quite real. What happens when that mechanism, whatever it is, fails to trigger, and such perceptions gain a foothold on the mind so that, as dreams sometimes do, they create pseudo memories so vivid that even shortly after waking we still feel they were real experiences? For a person with a crippled illusion detector, experience would require exhausting continuous sorting of perceptions into illusions or real events. It could be almost a full-time job; to get any peace, the person might need to let the distinction go.