I still can't agree to the premise that depression always has a biological basis (except for the obvious that humans are biological beings). No doubt there are some cases, but it seems to me frequently if somebody is depressed they might have an external reason for it.

Yes, I know about the measurements of brain activity, serotonin and whatever. That doesn't necessarily prove that people are depressed because of a malfunction - those things could as well be a result of the depression, rather than causing it.