Perhaps depression is simply hard to convey--even, as Styron says, "indescribable." But I'd like to suggest another possibility: That what we call "depression," like the mythical black bile, is a chimera. That it is cobbled together of so many different parts, causes, experiences, and affects as to render the word ineffectual and perhaps even noxious to a full, true narrative.
It is ironic, given the criticism directed at him, that Adolf Meyer seemed to have in mind the limits of single-word diagnoses when he proposed that depression replace melancholy. Meyer believed the former word, obviously inadequate, would force doctors to tailor their descriptions to individual cases. "Nobody would doubt that for medical purposes the term would have to be amplified so as to denote the kind of depression," Meyer wrote. (Italics added.) Perhaps Meyer even liked the insipid quality of "depression," believing it would announce (like a blank canvas or the blue screen on a film set) the absence of material to come.
If so, what transpired over the century can be counted among the great tragicomedies in the history of language: Somehow, we have come to believe that "depression" is the art, is the phantasm of special effects, is the evocative detail or phrase or story rather than a mere placeholder. The DSM-IV lists only a few qualifiers for "major depressive disorder." Psychiatrists and medical texts treat depression as a discrete entity, and assume it adheres to a particular course and treatment. Ads for drugs, herbal remedies, and nutritional supplements refer to depression as though it is a foreign invader, unrelated to the authentic self.
In lay culture, meanwhile, the word is often used with no context at all. A New York Times report on the rising suicide rate in Japan notes that the cause might be "depression," but does not offer even a single phrase to elaborate. In conversation, otherwise imaginative, articulate speakers toss around the words "depressive" and "depressed" as if they capture a person's essence. In his story "The Depressed Person," David Foster Wallace gives the eponymous character no other name, which I take as sardonic reflection on the way we drape over diverse sufferers a label that hides more than it reveals.