We know how the heart works, and what to do when something goes wrong. Why can’t we do the same thing with the brain when mental illness strikes? Good article here, which is a more applied version of the mind-body problem:
Even so, suppose that we knew at each instant what each neuron was doing – what chemical it was releasing and where. Suppose further that we could relate this to something the brain was doing at that moment (say, making you hungry, or seeing someone you knew) – a circumstance way beyond contemporary neuroscience. Would we really ‘understand’ what we were observing? Would we ‘know’ why this pattern represented a thought, a perception, a motivational or emotional state? Could we then predict what a different state of mind (say, thirst, or recognising a banknote) might require? There is no theory of neural function that would allow us to do this, beyond a vague generalisation that the particular activity of a neuronal assembly or network was responsible (and even this might vary in different parts of the brain). We don’t actually know what to look for.