The book "Horror in its relation to certain forms of medieval superstition. Papers in applied psychology. The fourth calculation" was written by Ernest Jones. In this work, the author examines the phenomenon of the nightmare, its relationship with psychology and the cultural context of the medieval period. This book opens up to the reader a deep analysis of the psychological aspects of fears and contradictions that accompanied people of that time, and also provides a unique perspective on the influence of superstitions on the mental state of an individual. Thanks to the research of Ernest Jones, the reader will be able to better understand the psychological mechanisms that governed people in the historical and cultural context of the Middle Ages.