In order to make fit my article 'Challenging Q' into the available
space, the editor of The Expository Times was obliged to cut a
couple of paragraphs and replaced them with the brief note 13. In case
anyone wonders why 'Foster's suggestion of a further asymmetry between the
reconstructions of Q and Mark in relation to Mark-Q overlaps is not
convincing', the text of the omitted passage is given below.
Foster suggests a further asymmetry between the reconstructions of Q and Mark in relation to Mark-Q overlaps:
In fact, it is obvious that not only the Beelzebul controversy, but also any passage assigned to the Mark-Q overlaps could not be recovered when we work back to hypothetical Mark. However, it is precisely because these passages do overlap with Mark that we have been able to identify them as potential Q passages. It is impossible to use such material to reconstruct 'Mark' but they are highly beneficial in determining the contents of Q. This again is another significant difference between the two cases that is not recognized. 
This point is correct if it is confined to the Mark-Q overlaps commonly recognized as such. But it does not constitute a 'significant difference between the two cases'. As a first approximation, the most generally recognized Mark-Q overlaps are those pericopae in which there is a major agreement of Matthew and Luke against Mark, explained by the hypothesis that both Matthew and Luke have made substantially more use of the Q version that its Markan parallel. In instances where the reverse was true, that is, where Matthew and Luke both made substantially more use of Mark than Q in a Mark-Q overlap passage, the most that would be observed would be a few minor agreements of Matthew and Luke against Q, and a Mark-Q overlap might not be suspected. These would be instances in which it was impossible to use such material to reconstruct Q although they would prove highly beneficial in determining the contents of 'Mark'. This possibility preserves the required symmetry between the two cases.
A fuller version of this somewhat compressed argument will appear in "Reconstructing Mark: A Thought Experiment"
 Foster, 'Defence', p. 298 n. 22
Last updated on 18-Sep-02
By Eric Eve