The book "Multiple Personality Disorder" is one of the earliest scientific studies to address multiple personalities as a psychological condition linked to a split in human consciousness. Published in 1904, it is based on a collection of clinical and psychological experiments conducted on patients who exhibited multiple personalities within the same body. Sidis posits that a person is not always a single, unified consciousness, but may consist of more than one "self" or "personality" that alternate in controlling consciousness, particularly in cases of psychological trauma or hypnosis. He explains how these personalities can arise as a result of a separation between the conscious and unconscious mind. The book is considered a classic reference in psychology and lays the foundation for understanding a disorder known today as Dissociative Identity Disorder (DID). It also sheds light on the relationship between hypnosis, suggestion, and split consciousness, and provides detailed descriptions of real pathological cases.