Thanks to the investigations of physiologist and psychologist, usually combined in the person of a physician, "nervousness" has been found to be not an organic disease but a functional one. This is a very important distinction, for an organic disease implies impairment of the tissues of the organ, while a functional disorder means only a disturbance of its action. In a purely nervous disorder there seems to be no trouble with what the nerves and organs are, but only with what they do; it is behavior and not tissue that is at fault. Of course, in real life, things are seldom as clear-cut as they are in books, and so it happens that often there is a combination of organic and functional disease that is puzzling even to a skilled diagnostician. The first essential is a diagnosis as to whether it is an organic disease, with accompanying nervous symptoms, or a functional disturbance complicated by some minor organic trouble. If the main cause is organic, only physical means can cure it, but if the trouble is functional, no amount of medicine or surgery, diet or rest, will touch it; yet the symptoms are so similar and the dividing line is so elusive, that great skill is sometimes required to determine whether a given symptom points to a disturbance of physical tissue or only to behavior.