Medieval Theories

Abstract

Many medieval writers argued that sleep was caused by corporeal processes related to digestion. This theory was largely transmitted to the medieval West through the medical treatise Pantegni which considered the vapours of digestion rising to the brain as the cause of termination of the sensory operations in sleep. While this explanation was later supported by Aristotle’s works, it was in most cases based on a brain-centered view of perception (1). Some medieval authors elaborated the ancient classifications of types and causes of dreams. An anonymous twelfth-century treatise Liber de spiritu et anima disseminated Macrobius’ classification of dream-content into the following five categories: oracular saying, vision, dream, nightmare, and apparition. Other writers like Averroes provided psychological explanations based on standard notions of internal perceptive faculties (2). Among various kinds of dreams, the nature of prophetic dreams was discussed in particular. Thomas Aquinas thought that both God and natural agents, such as separate substances, cause prophetic dreams (3). The similarities and dissimilarities between cognition in dreaming and in external perception were also discussed by many authors (4).