This long essay from The New Republic put me to sleep before I was half way through but it looks very informative. It also namechecks one of my favourite books:
In emphasizing the "caused" versus "non-caused" aspect of depression, Horwitz and Wakefield seek to revive a once-vital concept. As they document in rewarding detail, the importance of context as a key to whether a condition is abnormal was appreciated throughout the ages. Hippocrates and Aristotle distinguished melancholic states -- considered a surfeit of black bile in those days -- according to whether they arose with or without cause, associating only the latter with disease. Roman physicians also assented to this distinction. In the Renaissance, even greater emphasis was placed on cause. In 1621, Robert Burton, the author of the great Anatomy of Melancholy, identified today's equivalent of depressive sickness as "sorrow...without any evident cause grieving still, but why they cannot tell."