It's not a matter of belief. It's a fact.
The human mind can cause physical symptoms.
If someone who is suffering from paranoid schizophrenia thinks they are being shot in the eye by a
laser, pain can manifest in the their eye.
We can tell them that it isn't really happening but it doesn't really matter that it isn't happening
when it is real to them.
I'm not quite sure of the question, exactly. If it is: Does the mind have some control over the
body, then I would say, "most certainly." If the question is: Can the mind control the physical
world around us, then I would say that the evidence says, "no."