First McGrath. Then Goodacre, and now back over on Exploring Our Matrix, James McGrath offers some more thoughts on how the differences make Q. I must disagree.
Why, for example, would we assume that Luke, once he'd decided to alter Matthew's genealogy, would bother to laboriously copy "more of the names" the same? That seems remarkably tedious to me, I probably couldn't be bothered were I him.
The reason for that is probably my biggest problem with McGrath's line of reasoning: It requires us to assume that Luke was naive. If Luke wasn't naive, he probably couldn't be bothered either.
Surely if Luke sat down with Matthew and Mark in front of him, fully intending to redact them in to something new, he would be fully aware that Matthew had done the same. The Exodus symbolism is patently obvious to us, for example, why should we assume that Luke missed it? And if he didn't, why would he regard it as authoritative in the sense that he does Mark?
If, continuing with the Exodus example, Luke rejects Jesus as the second Exodus--even symbolically--why would he wish to keep Matthew's infancy, knowing full well what Matthew has created? There doesn't seem to be much room in Luke's gospel for a second Moses--this would seem to be a reasonable position to take.
Even beyond Lukan sympathies, surely some of Luke's narrative was influenced by a knowledge of historical realities. While I have no pretenses about "Luke the Historian," it's easy to see, for example, why he might omit the slaughter of the innocents: He knew it wasn't true. If you were Luke, and realized that, how seriously would you take Matthew's narrative? How much authority would you grant it?
And there are points of similarity between the infancies. Perhaps most notably in the case of the virgin birth. The virgin birth fairly reeks of Matthean creation to me, so how does Luke know it? We might get around this by suggesting that Luke had received oral tradition that was shaped by Matthew, but now we've just moved the problem back. Surely Luke didn't hear only the virgin birth, so why did nothing else he received orally make the cut? Why did he revise that oral tradition? Q doesn't solve that difficulty. The differences don't make Q, Q just makes the differences somebody else's problem.