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The placebo effect is when a patient believes that a drug is effective when it is not. This is also the use of illusion.
On the other hand, if you blindfold them, make a small wound on their arm, and drop a drop of water on them, they say that they feel that their bleeding will not stop. As a result, they say that the person only had a small wound on their arm and no bleeding at all, but they were on the verge of death.
Illusion can save people or kill them. The old saying comes to mind. It all depends on your mind.