On the face of it, the options are these. (1) Maybe minds are what brains do, and ultimately physical in origin. (2) Maybe minds are immaterial spirits of some kind, and brains are just a distraction. (3) Maybe minds are immaterial spirits of some kind, and brains are how they interface to the material world.

The clearest evidence distinguishing these -- which heavily favours #1 over #2 and #3, as it turns out -- comes from seeing what happens when a brain is permanently or temporarily impaired by physical damage, drugs, oxygen shortage, electric shock, etc.

If #2 were right, those things shouldn't affect your mind at all; but of course they do, so #2 is out. So dualists generally prefer some version of #3: the mind is no more created by the brain than a concert is created by the radio you hear it with, even though destroying the radio stops you hearing the concert.

But now imagine that you have an electronic box that plays music, and it turns out that if you damage it in one way then the violins all start playing out of tune, and if you damage it in another way then the music is all OK except that the singers go la-la-la instead of singing the right words, and if you damage it another way then every time you play sad music it stops working for a while. And another kind of damage leaves all the music kinda correct, but it's performed mechanically and boringly and emotionlessly. And another kind means that whatever you do to it it only performs sad 19th-century German songs about love and death. You'd quickly come to the conclusion that it isn't just a radio; it's somehow generating the music itself.

The effects on the mind of messing physically with the brain are rather like that. Injuries in particular places can cause very specific kinds of disability -- a patient who's mostly OK but can't remember the names of any vegetables, or another who has lost the use of one arm but also lost the ability to recognize that that arm doesn't work, etc., etc., etc. It's all exactly what you'd expect if mental functions are implemented by the brain and damaging particular bits of brain damage particular mental functions.

It's not a theorem; none of this is quite proof. It could be, e.g., that your mind is operated by an angel who watches for damage to your brain and interferes with the mind in just the way that the brain damage would produce if the mind were just a product of the brain. But it's that kind of hypothesis you end up needing, if you seriously attempt to reconcile the observable facts with a belief that minds are separate from brains.

Many of them are so inert, so hopelessly dependent on the system, that they will fight to protect it.