The Psychology of Insanity by Bernard Hart is a landmark text in early 20th-century psychiatry, offering one of the clearest introductions to mental illness for both medical students and general readers of its time. First published in 1912 and reissued in later editions, the book reflects the growing influence of psychology and psychoanalysis on psychiatry while retaining a strong clinical perspective. Hart explains the nature of insanity not as a mysterious affliction but as a condition that can be studied, classified, and understood in terms of psychological principles. He discusses symptoms, causes, and types of mental illness, ranging from delusions and hallucinations to hysteria and dementia. Importantly, he frames these conditions within the broader context of human psychology, emphasizing continuity between normal and abnormal mental processes. Written with unusual clarity and balance, Hart's book became widely respected for making complex psychiatric concepts accessible without oversimplification. He draws on clinical cases, contemporary research, and his own experience to illustrate how mental disorders affect perception, thought, and behavior. While inevitably dated in terminology and theory, The Psychology of Insanity remains historically significant as an early attempt to bridge psychiatry, psychology, and the lay understanding of mental health. Today, it offers modern readers not only insight into the evolution of psychiatry but also a window into how early 20th-century medicine sought to destigmatize mental illness by explaining it in rational, human terms.