Original sentence: He has also written about religious experiences, defending a four-stage model of religious experiences, based upon Graham Wallas' four-stage model of problem-solving in the psychology of creativity
Edited sentence: He has also written about religious experiences, defending a four-stage model of religious experiences, based upon Graham Wallas' four-stage model of problem-solving in the psychology of creativity (Batson, Schoenrade & Ventis, 1993)