This is something we talk about, but rarely think about: I can give you an inert substance that still heals you, just because you believe it will work or want it to. Think about that a second. It shouldn't work medically/scientifically, but it does, and is well documented. That means mind over matter works in medicine: and we're not sure why.
There's also the counter to the placebo, but the nocebo -- which is an imagined toxin or negative effecting compound.