In my first post on this topic, I suggested that the term "Mark Q Overlap" is problematic because it names the data set by using a proposed solution to the problem apparently posed by that data set. I would now like to begin to explore why alleged "Mark Q overlaps" cause problems for the Two-Source Theory that are not often noticed.
In comments to an earlier related post, an anonymous commenter (please sign your name) criticized me for speaking too in-house, for not writing for "non-specialists". There is, of course, a lot of truth in that. I can't re-introduce the Synoptic Problem for non-specialists every time I blog about it. But I value the reminder of the importance of being as clear as possible, so in what follows, I will try to state things as clearly as possible, trying to lay out steps along the way. Inevitably, though, the discussion will make more sense the more familiar one is with Synoptic discussions.
2. Major Agreements between Matthew and Luke: the Continuum
Mark Q Overlaps are more appropriately designated "major agreements between Matthew and Luke against Mark". The value of this term is that it is neutral, and describes this particular set of interesting data. This set of data might also be described as triple tradition material in which Mark is not the middle term, or triple tradition material in which there is extensive agreement between Matthew and Luke against Mark. Why is this set of data interesting or important?
Normally speaking, the pattern in triple tradition material (Matthew // Mark // Luke) is that there is a lot of agreement between Matthew and Mark alone, a lot of agreement between Mark and Luke alone, a lot of agreement between all three, but only minimal agreement between Matthew and Luke. In other words, Mark is usually the middle term. Marcan Priorists explain this by suggesting that Matthew and Luke were both reliant on Mark for this material. So far the Two Source Theory and the Farrer Theory, both Marcan Priority theories, are united.
However, there are many places where this normal pattern of agreement is not the case, and where Mark ceases to be the middle term. These are the passages under discussion here, the so-called Mark Q overlap passages, where there is substantial agreement between Matthew and Luke against Mark. For the Farrer theory, according to which Luke is familiar with Matthew as well as Mark, these passages present no difficulty. They are places where Luke is primarily dependent on Matthew, where he turns to Matthew's account rather than Mark's, generating much higher levels of agreement with Matthew than with Mark. For the Two-Source Theory, according to which Matthew and Luke are independent of one another, these passages might, at first glance, appear problematic. How can Matthew and Luke be agreeing so extensively against Mark when they are dependent on Mark for their triple tradition material?
The Two-Source Theory solves the problem presented by these passages by suggesting that the two sources of Matthew and Luke occasionally overlapped. On such occasions, Matthew and Luke turned to the hypothetical source Q instead of or as well as Mark, thereby generating major agreements between one another and against Mark. That two sources should have overlapped in this way is, in itself, quite plausible. Perhaps both Mark and Q each told similar versions of the same events, recording similar versions of the same speeches. This tidy solution, however, masks some serious problems for the Two-Source Theory that often go unnoticed.
One of these problems has to do with the reasons offered for postulating Q in the first place. One of the standard arguments offered for the existence of Q is that Matthew and Luke never agree with one another against Mark in any substantial way. It is said that the agreements between Matthew and Luke are "too minor" to show any direct link between them. The existence of these major agreements, the so-called Mark Q overlaps, contradicts these kinds of assertions. And this is why the naming of sets of data is so important. When this material is categorized as "Mark Q overlap", it effectively hides it from view when scholars look at the triple tradition overall. They only see the triple tradition that features minor agreements, and regard the minor agreements as not significant enough to make the case for a direct link between Matthew and Luke. (The many, many Minor Agreements are significant enough to make the case too, but that is another story).
There is a related problem that the categorization here causes. By boxing the different degrees of agreement between Matthew and Luke into separate categories, we fail to see that there is in fact a spectrum of agreement between Matthew and Luke which can be crudely illustrated like this:
Here the horizontal axis represents the influence of Mark on Luke and the vertical axis represents the influence of Matthew on Luke. What we have is a continuum, from lesser to greater degrees of agreement between Matthew and Luke. The continuum ranges from pure triple tradition to pure double tradition, with varying degrees of agreement along the way, from relatively minor to quite major agreement between Matthew and Luke against Mark. It is a pattern that makes good sense on the assumption that Luke uses Mark, but supplements his use of Mark with his use of Matthew, sometimes in minor ways, sometimes in major ways. The Mark Q overlaps are not a separate category of data to be separated off and explained in a unique way. They are, rather, points that one can plot on a graph, mid points in a spectrum.
(See further my Fallacies at the Heart of Q and, in greater detail, my article A Monopoly on Marcan Priority? Fallacies at the Heart of Q; see too The Case Against Q, chapter 3).