Both Pierre Janet and the neo-Janetian contemporary theory of structural dissociation of the personality (SDP) view dissociation as inherently pathological – trauma-related dissociation. However, since the late 19th century, other theories have viewed dissociative subsystems of the personality developing after traumatic experiences as continuous with proposed divisions of normal personality. Taking Pierre Janet's hierarchy of degrees of reality as a guide, along with the basic premises of the theory of structural dissociation of the personality, this paper examines this assumption in constructs from the late 19th through 20th centuries, including ego states, self-states, schema modes and complexes. It is concluded that the SDP concept of dissociative parts of the personality is most consistent with the historical and empirical literature, and that dissociation is best thought of as discontinuous with normal personality.