I think it’s pretty clear that “depression” is a normal psychological state, and that it is functional. It would be surprising to find such a common, and profound, behavior in a long running, successful species if it wasn’t.

As the example of learned helplessness illustrates, depression can be induced in a predictable way. Even on an intuitive level, a stance of submissive apathy could increase chances of survival, if one is a captive.

My (lay person’s) view is that what we see as clinical depression is a symptom of a “bias” or miscalibration issue, where the feedback circuits in the brain responsible for maintaining homeostasis have been forced to work outside of their functional ranges. The brain has a lot of redundancy and compensating routines, but they have their limits.

One of the most obvious causes would be having a weakened part of the brain, whether through injury, genetic, or environmental causes. The brain becomes miscalibrated, and can’t maintain equilibrium because some essential component is operating “out of spec.”

If you look at other human attributes and biological systems, they are all prone to disorder. Or perhaps, to having characteristics that fall on the outer edges of the distribution of the characteristic. As a simple example, when an individual is under a certain height, we call that a “disorder” or “syndrome.”

So there is an element of arbitrariness and imprecision as to where these definitional borders are drawn. They are fuzzy. But, looking at a larger population, you can start to see patterns where you can definitely place an individual into a sub population of people who have a disorder.

Mania, for example, has enough common symptoms, that I am confident it is an actual, physiologically based, “disorder. Even though the symptoms of mania are all behaviors and states experienced by “healthy” people, all the time.

I also have a theory that by the time a mental health problem starts to become perceptible by others, it is actually a pretty severe condition. The individual’s internal state is probably far more disordered than it appears to be. Because our brain is so flexible, and powerful, it can compensate even extremely disorder functioning, for the purpose of hiding the illness from others.

My ideas are largely consonant with yours. You’ve outlined a “model”of depression, which uses the concept of depression being a response to having ones mental construct of reality broken. That would be the functional aspect of depression.

My view is that “depression” the disease occurs when the brain can no longer maintain homeostasis. So if someone is being tortured, for example, depression would be a an expected response. If the torture is severe enough to push past the ability of the brain to maintain order, which is the point of torture, that’s a situation where calling the state a “disease” or an “expected response” is simply a matter of semantics.

Because depression is a normal state, part of a stress response, it’s symptoms can look, and be, very similar, even though the causes can be myriad. I think this is why the blunt instruments of psych drugs are used to treat these symptoms across so many different scenarios. They don’t work great, but they do work.