The question of depression …

… Melancholic Mysteries . (Hat tip, Dave Lull.)

One of the problems of depression as an object of study is that there are no clear-cut biological markers to distinguish cases from non-cases. Some endocrinological conditions mimic depression, and some medications undoubtedly cause it. In the great majority of cases, however, there are no measurable bodily changes, except those brought about by the symptoms themselves: in depression, symptoms and disease are one, at least in the present state of knowledge.