In connection with will theory and other aspects of medieval psychology, one wonders why not only St. Thomas but all the great medieval writers quote St. John Damascene's De Fide Orthodoxa.¹ It is the purpose of this discussion to try to explain two aspects of this source: (1) the precise contribution that the Damascene makes in regard to will theory, and (2) the origin of this element of will theory for John Damascene himself. There are at least two other facets of this problem that I leave for a later discussion, namely (1) how the medievals became acquainted with the thought of Damascene and (2) a further implication of Damascene's doctrine on the will in relation to the establishment of the moral "ought."²