This study examines the history of a fundamental problem in Aristotelian cognitive psychology, ie the nature and function of the mechanisms that provide the human mind with data concerning physical reality. The first chapter traces the classical and Arabic prehistory of the mediaeval doctrine of intelligible species. Scholastic discussions on formal mediation in intellective cognition were constrained in essential ways by Thomas. The work then analyzes his views on mental representation in the context of the reception of Peripatetic psychology in the West. The following chapters examines the controversies about the necessity of intelligible species, from Aquinas' death to the 15th century. Volume 2 will be devoted to Renaissanc discussions, developments of later scholasticism, and the elimination of the intelligble species in modern non-Aristotelian philosophy.