It is a marvelously true expression. _Whatever_ the cause, a real black-dog depression robs a person of all sense of effective agency. The leg is broken and needs a cast, and for many the drugs do work (personal experience). But then the person needs to learn to walk again. It may be a disease (there is a genetic component) but we don't know whether the observed neurotransmitter problems are cause or effect.

Freud said that the purpose of therapy was to replace neurotic misery with ordinary human unhappiness. Some Californians find that a bit of a downer, but it's true, and we can do something about unhappiness.