In contemporary discussions of the NT canon, focus has been on its polemical aspects, that is, when it was closed. By so doing the idea of a canonical process suffers. In attempting to understand Tatian's Diatessaron in this process it is argued here that the very existence of the harmony testifies against a closed fourfold Gospel canon in the mid–second century. A proper distinction between canon and scripture is foundational in this understanding. Discussions about the closed NT canon belong to a day far removed from Tatian's. By placing Tatian's Diatessaron in the perspective of process we are less tempted to view his use of the four Gospels as proving their canonicity, a view which is anachronistic and inaccurate.