I need to work on briefer posts, so that I can be more consistent here. The blogger as aphorist. Nothing so pithy today, but on Arts & Letters Daily today I found a book review by Sally Satel that pretty much summarizes what psychiatry is about. Read the article yourself--isn't that what makes web links so wonderful, that they obviate the need for dull summaries?
Mental disorders are disorders of agency and intentionality, which is not to say that they are simply chosen or merely willed. That is why psychiatry cannot and should not merge with neurology, which deals primarily with the nervous system as machine. As Satel notes, the crucial difference is the response of mental disorders to contingencies, which gives them a social and semantic distinction from purely physical disease states.
This is not to say that there is a clear and absolute dividing line between mental and non-mental disorders, but the distinction is there nonetheless. Inasmuch as they pertain to ailments of identity and free will, mental disorders are disorders of the self, which may sound grim and slanderous until one remembers that "ailments" range from the blemishes that we all have to, well, the cancers of the soul (which is the brain--how complicated!).