Depression, when it's clinical, is not a metaphor. It runs in families, and it's known to respond to medication and to counseling. However truly you believe there's a sickness to existence that can never be cured, if you're depressed you will sooner or later surrender and say: I just don't want to feel bad anymore. The shift from depressive realism to tragic realism, from being immobilized by darkness to being sustained by it, thus strangely seems to require believing in the possibility of a cure.