A remarkable study was done on schizophrenia a while ago. One case involved a woman with a spilt personality.

Normally her blood sugar levels were completely normal. But when she believed she was a diabetic in one of her split personalities, her whole physiology changed to become that of a diabetic. This change was measured by doctors present.

Her belief became her reality. Because she totally believed, without doubt or question , this belief changed her entire physiological state.

Although this may sound extraordinary, this case is true and is well documented.

In fact, you may have seen numerous hypnotic demonstrations where a person is touched with a piece of ice which is represented to him as a piece of boiling hot metal. 

A belief is planted in his mind that this piece of ice is in fact a piece of hot metal.

If the subject accepts this belief - invariably a blister will develop at the point of contact. Yes - A real physical blister!

Can you see what is happening here ? 

What counted was not the reality of the situation but the belief - the subject's brain and nervous system believed that the skin was being touched by a hot metal rod and acted accordingly.

Even though this was not true - it makes no difference - because by believing it was true, he responded as if it was true.