In the study of _Love and Pain_ I have discussed the sources of those aberrations which are commonly called, not altogether happily, "sadism" and "masochism." Here we are brought before the most extreme and perhaps the most widely known group of sexual perversions. I have considered them from the medico-legal standpoint, because that has already been done by other writers whose works are accessible. I have preferred to show how these aberrations may be explained; how they may be linked on to normal and fundamental aspects of the sexual impulse; and, indeed, in their elementary forms, may themselves be regarded as normal. In some degree they are present, in every case, at some point of sexual development; their threads are subtly woven in and out of the whole psychological process of sex. I have made no attempt to reduce their complexity to a simplicity that would be fallacious. I hope that my attempt to unravel these long and tangled threads will be found to make them fairly clear. In the third study, on _The Sexual Impulse in Women_, we approach a practical question of applied sexual psychology, and a question of the first importance. No doubt the sex impulse in men is of great moment from the social point of view. It is, however, fairly obvious and well understood. The impulse in women is not only of at least equal moment, but it is far more obscure. The natural difficulties of the subject have been increased by the assumption of most writers who have touched it--casually and hurriedly, for the most part--that the only differences to be sought in the sexual impulse in man and in woman are quantitative differences. I have pointed out that we may more profitably seek for qualitative differences, and have endeavored to indicate such of these differences as seem to be of significance. In an Appendix will be found a selection of histories of more or less normal sexual development. Histories of gross sexual perversion have often been presented in books devoted to the sexual instinct; it has not hitherto been usual to inquire into the facts of normal sexual development. Yet it is concerning normal sexual development that our ignorance is greatest, and the innovation can scarcely need justification. I have inserted these histories not only because many of them are highly instructive in themselves, but also because they exhibit the nature of the material on which my work is mainly founded.