I do agree that depression can and does exist even with a sense of purpose and I agree that depression is in large part due to a chemical imbalance in the brain.
But I also believe it's possible for a person to lose or struggle with a sense of purpose due to a variety of external events and thus over a prolonged period of time experiencing the stress of such, along with a gradual change in said person's thought process, partly by choice, partly sub conscious reaction, can eventually induce depression due to the brain's neurological architecture being altered, but I belief the opposite is also true, gradual healing from therapy, supportive enviroment and conscious and deliberate changes in thought process could alter the neurological architecture of the brain to become more stable again.
But I do believe that inspite of all that, the causal factor behind the gradual change could be beyond a person's control and might require medication or other means to help the brain function properly, beyond the above listed things.
Hence enviromental, and clinical depression.
The point I was trying to make was the possibility of depression being induced by a change in thought process over a period of time, or perhaps more accurately put, the possibility of a depressive state being induced as opposed to clinical depression.
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Christian, Aspergian, Recovering Bundle Of Neurotic Anxieties.