How Much Of What You See Is A Hallucination?

We all know that our eyes can be fooled by optical illusions, but how confident can we be about the ordinary world around us? The answer isn’t as simple as it might appear. The world we see is created by the mind from incomplete information, and there’s no way for us to be certain that isn’t adding information elsewhere without us noticing. Well, not unless we happen to be sure that what we’re seeing isn’t real:

An elderly woman named Rosalie was sitting in her nursing home when her room suddenly burst to life with twirling fabrics. Through the elaborate drawings, she could make out animals, children, and costumed characters.

Rosalie was alarmed, not by the intrusion, but because she knew this entourage was an extremely detailed hallucination. Her cognitive function was excellent, and she had not taken any medications that might cause hallucinations. Strangest of all, had a real-life crowd of circus performers burst into her room, she wouldn’t have been able to see them. She was completely blind.