The fear of the unknown, of the unfamiliar, of the mysterious, is of the utmost consequence in the life history of children, savages, and barbaric tribes, and even in the social life of civilized nations. The fear of coming mysterious, unknown evil is a source of great anxiety to the young, or to the untrained, uncultivated minds. All taboos of primitive societies, of savages, of barbarians, and also of civilized people take their origin, according to anthropological research, in the perils and salvation of the soul, or in the fear of impending evil. As an anthropologist puts it: “Men are undoubtedly more influenced by what they fear than by what they love.” The civilized nations of antiquity used to be terrorized by omens, by occurrences of an unfamiliar character, such as storms, thunders, lightnings, comets, meteors, meteorites, and eclipses. Affairs of states and wars were guided by superstitions of fear. Whole armies used to throw away their weapons and run panic-stricken at the appearance of meteorites, meteors, and especially of comets. Even the ancient Athenians were influenced by strange, meteorological phenomena. On the appearance of a solar eclipse Pericles saved his ship by throwing his mantle round the helmsman, telling him that that was all that an eclipse was, and that there was no reason to be scared by the veiling of the sun from us. The father of pragmatic history, the great Thucydides, in his history of the Peloponnesian wars, puts the appearance of comets among national disasters. The fear of the mysterious, the unknown, and the unfamiliar is a source of anxiety and distress in the young, or in the untrained and uncultured minds. Fear may become fixed and morbid when taking place in early childhood, when not inhibited by the course of further development, and, all the more so, when kept up by further events of life. In most people the instinct of fear is controlled, regulated by education, and inhibited by the relatively secure life led in the herd, pack, group, and society generally. The instinct of fear, however, is but dormant and requires the opportune moment such as a social, mental epidemic, a “group-panic,” to become manifested in its full intensity, giving rise to a morbid state of the “group-mind,” or “herd-mind.” There are again cases when even under ordinary conditions fear becomes developed in the individual from early childhood either by lack of inhibitory training or by accidents in early child life. In all such cases the fear instinct becomes morbid, giving rise in later life to various forms of mental disease known as psychopathies, or recurrent morbid states. We can, therefore, realize the full significance of the principle laid down by one of the greatest thinkers of humanity, Plato, that to learn “What to fear and what not to fear” is of the utmost consequence to the individual, both in his private and social activities.