The present volume contains three studies which seem to me to be necessary _prolegomena_ to that analysis of the sexual instinct which must form the chief part of an investigation into the psychology of sex. The first sketches the main outlines of a complex emotional state which is of fundamental importance in sexual psychology; the second, by bringing together evidence from widely different regions, suggests a tentative explanation of facts that are still imperfectly known; the third attempts to show that even in fields where we assume our knowledge to be adequate a broader view of the phenomena teaches us to suspend judgment and to adopt a more cautious attitude. So far as they go, these studies are complete in themselves; their special use, as an introduction to a more comprehensive analysis of sexual phenomena, is that they bring before us, under varying aspects, a characteristic which, though often ignored, is of the first importance in obtaining a clear understanding of the facts: the tendency of the sexual impulse to appear in a spontaneous and to some extent periodic manner, affecting women differently from men. This is a tendency which, later, I hope to make still more apparent, for it has practical and social, as well as psychological, implications. Here--and more especially in the study of those spontaneous solitary manifestations which I call auto-erotic--I have attempted to clear the ground, and to indicate the main lines along which the progress of our knowledge in these fields may best be attained.