A group of problems that appears conspicuously in the present volume, and in so far contributes to the fitness of its title, has obtained a considerable interest on the part of the public at large. Such interest seems prone to take its clue from the activity of those who herald startling revelations on the basis of unusual psychic experiences, and who give promise of disclosing other worlds than the one with which common sense and common sensation acquaint us, rather than from the cautious and consistent results of serious and professional students in study or in laboratory. The fascination of the unusual over the popular mind is familiar and intelligible, and seems in no direction more pronounced than in matters psychological. So long as this interest is properly subordinated to a comprehensive and illuminating general view of the phenomena in question, it is not likely to be harmful and may prove to be helpful. But when the conception of the nature of our mental endowment and the interest in the understanding thereof are derived from the unusual, the abnormal, and the obscure, instead of from the normal, law-abiding observations systematized and illuminated by long and successful research, there is danger that the interest will become unwholesome and the conception misleading.