Okay, agreed, they're not separate entities, there's really only a body. The "mind" is not a "thing" but just a collection of capabilities and activities of one bit of the body, mainly the brain. A bit like your knee being able to bend. Bending your knee can affect other bits of your body (like the blood rushing to that area from other parts). And so can using your brain (e.g., if you're frightened, it can initiate the production of chemicals that then circulate through your body).
So let's see how that goes, now thinking in a totally non-dualistic way. Not separating the mind from the body at all. What's causing, say, cancer now? Could it be some cells are reproducing in your body that your immune system should have killed but missed (cos it doesn't work perfectly 100% of the time, or because it was busy at the time fighting off some infection or whatever)? Would that seem plausible?
Now let's try the psychogenic view, again not separating mind and body. Maybe you were thinking really negative thoughts or having really negative feelings, and they were so negative that your brain somehow ended up producing too much of some chemical (maybe a glucocorticoid, for argument's sake), and this reaction was intense enough and sustained enough that it messed up your immune system, and caused it to miss the cancer.
What you notice is the first explanation is really elegant and simple. The second one is much more complex. You'd need a really good reason to prefer the second over the first. You'd need strong evidence. The other thing you notice is that for the second explanation, you have to assume the brain has quite extraordinary and sustained ability to modify other body systems. I'd wanna see evidence of this too.
So, I guess I'm just playing thought games. But I just wanted to point out that the claim that "you can't separate the mind from the body" can't just be usedwilly-nilly to justify psychological explanations for illness. It comes at a price. It requires that we talk about these things in a truly non-dualistic way.
(PS. This is in no way intended as a criticism of you
@amaru7. What you said just got me thinking about the whole thing. Thanks for posting.)
rant over.....