The Rigid Rule of All-or-nothing Factual Independence

Today I'm discussing another aspect of the rigidity of critical scholarship: Among the Synoptic Gospels, they assume that if Matthew and/or Luke is "using Mark" for a story, then we shouldn't take seriously the possibility that Matthew and/or Luke is also supplementing Mark from additional factual information to which he has independent access. Factual independence is treated as all-or-nothing. So if Matthew and/or Luke differs from Mark, this must be from some way in which he is changing Mark, without factual input--making up details for which he has no evidence, factually altering the story, etc. But that's not how real-life testimony works. It can be partly dependent and partly independent. This realization is a game-changer in considering the so-called Synoptic problem.