The author extends Hallowell's interest in dreams by offering an analysis of altered states of consciousness (ASC). She argues that, like dreams, ASC have evolved in human development, with roots in mammalian biology. Their significance in human life derives from the symbolic transformation of experience and the capacity to share intrapsychic states. Unlike dreams, ASC derive from models based on pathological states. Examples are offered from the case of Hildegard von Bingen, as well as from ethnological studies. The author concludes that the foundation of ritualized altered states lies in the development and modification of the group's “behavioral environment.” They therefore serve as coping mechanisms for both the individual and the society and thus provide a basis for culture building.