Kraepelin: manic-depressive insanity and paranoia Watch
Anyway, Kraepelin was a German psychiatrist who conceptualised severe mental illness/psychosis (including schizophrenia, bipolar, true melancholic depression) in a way that no one else could dream of. Problem his mainstream psychiatry's gone the completely wrong way and the classical descriptions are mostly unknown to the ignorant general public but here's his book on bipolar disorder.
I just feel that people would never question the validity of genuine psychiatric depression if they read how it is in its greatest form. And hundreds of years ago, not even as a recent label.