It may surprise many medical readers that in the third and longest study I have said little, save incidentally, either of treatment or prevention. The omission of such considerations at this stage is intentional. It may safely be said that in no other field of human activity is so vast an amount of strenuous didactic morality founded on so slender a basis of facts. In most other departments of life we at least make a pretence of learning before we presume to teach; in the field of sex we content ourselves with the smallest and vaguest minimum of information, often ostentatiously second-hand, usually unreliable. I wish to emphasize the fact that before we can safely talk either of curing or preventing these manifestations we must know a great deal more than we know at present regarding their distribution, etiology, and symptomatology; and we must exercise the same coolness and caution as--if our work is to be fruitful--we require in any other field of serious study. We must approach these facts as physicians, it is true, but also as psychologists, primarily concerned to find out the workings of such manifestations in fairly healthy and normal people. If we found a divorce-court judge writing a treatise on marriage we should smile. But it is equally absurd for the physician, so long as his knowledge is confined to disease, to write regarding sex at large; valuable as the facts he brings forward may be, he can never be in a position to generalize concerning them. And to me, at all events, it seems that we have had more than enough pictures of gross sexual perversity, whether furnished by the asylum or the brothel. They are only really instructive when they are seen in their proper perspective as the rare and ultimate extremes of a chain of phenomena which we may more profitably study nearer home.