Abstract
Popper and Eccles present two different notions of Interactionism. Popper's arguments arise out of the traditional philosophical debate, whereas Eccles' arguments arise out of a mixture of neurophysiology and personal belief. Popper's three-world ontology is the philosophical foundation of both their positions. However, it is precisely against the background of the three Worlds that the considerable differences between their positions are apparent. Despite these defects, Interactionism is a productive notion since it does not place the Self beyond experimental investigation. Indeed, both Popper and Eccles have made implicit suggestions as to experimental procedure which might contribute to the investigation of the Self.