Well neural computations happen at the level of individual neurons up through complex multi-neuronal circuits, cognition and behavior are the result of neural computation, therefore if cognition is abnormal then computations at the level of neurons up to circuits must be deranged. Nothing fancy.
>cognition and behavior are the result of neural computation
That's like saying if someone gets shot and dies it's because they merely had a faulty brain -- it's ignorant of the actual cause.
Neurons don't exist in a vacuum, they respond to social, environmental, and biological inputs.
How does psychiatry scientifically decide if a persons deranged mental behavior is due to mere deranged circuits or deranged social, environmental, or biological inputs?
I see where you’re coming from. I think a great deal of mental illness is social and environment. We did not evolve to live in the circumstances of developed societies. So absolutely if we could change environment and social factors that would help a lot.
But just take depression for example. Let’s say Bob goes to a psychiatrist with depression. Bob recently lost his job and his girlfriend left him. He has every reason to be depressed. He would probably feel a lot better if he had a job he likes and wasn’t so alone. The problem is he’s too depressed to look for a job or to go out and meet new people, the things that would help his depression. This is where I think psychiatry can be helpful; some people just need something to break them out of their depression for long enough to be able to make the social/environmental changes in their life that will help them in the long run. Therapy alone may be enough for some people but not for everyone.
>some people just need something to break them out of their depression for long enough to be able to make the social/environmental changes in their life that will help them in the long run
