[E]ven though puzzle pieces can be forced together in countless ways, they should not be arranged in the ill-filling configuration mythicism offers, when other arrangements allow those pieces we have (many must be presumed to be missing) to be fit together without the use of scissors. Dr. James McGrath
Dr. James McGrath of Butler University is blogging about Earl Doherty's Jesus: Neither God Nor Man: The Case for a Mythical Jesus at Exploring Our Matrix. I have never read any of Doherty's books, but he is generally regarded as making made the strongest possible case for Jesus mythicism (however that term is defined). In the past, Dr. McGrath has opined that the strongest possible case for mythicism is still worthy of little more respect than the strongest possible case for creationism and nothing in his first couple posts on Doherty's book suggests that he is going to change his mind about it.
McGrath may be correct about the shortcomings of mythicism, but I think he underestimates the extent to which the same sorts of problems plague historicism. Not only must we presume that many of pieces of the puzzle are missing, I think we can be confident that the overwhelming majority of the pieces are missing. Nor do we know how big the puzzle is so we don't know whether the handful of pieces we have is any sort of meaningful fraction of the total. On top of that, we cannot be sure that all the pieces we have belong to the same picture. Some of them may have been put in our box by mistake.
McGrath may be correct that the historicist's arrangement of the pieces is more plausible than the mythicist's, but I can't help but think that the advantage may be trivial. Sticking with the jigsaw puzzle analogy, we have some pieces that are light blue and some that are dark green. It may be the best guess that the blue pieces are part of the sky and the green pieces are parts of trees, but it is still just a guess. There are lots of other objects that can be blue and lots of other objects that can be green. The person who guesses sky and trees may have better odds of being right than the person who guesses house and car who may have better odds of being right than the one who guesses cat and dog, but that doesn't mean that any of them are justified in thinking that they know what the whole picture looks like.
Of course no historian has all the pieces to any puzzle and the problem only becomes more acute for ancient events, but I believe that some historians of ancient times have things better than historical Jesus scholars. A scholar who wishes to examine how a particular war unfolded has much more to guide him in arranging his pieces. There is archeological data that can be consulted. There are accounts of other ancient battles and wars that can be used for comparison. Things that are known can be used to help form hypothesis about things that are unknown. On the jigsaw puzzle analogy, perhaps it is like having some edge pieces that at least define the boundaries and size of the puzzle.
For the historical Jesus scholar, on the other hand, there seems to very little useful data concerning the origins of ancient religions. We know something about what people believed at various times and places, but very little about how they came to believe it. Dr. McGrath argues that the existence of an actual historical Jesus is a much more likely explanation for the origin and growth of a religion based on a crucified Davidic Messiah than a purely mythical or legendary Jesus could be, however, it is hard to see what the basis might for assessing this probability. We simply don't have a basis for comparison. Indeed, the data we do have suggests that people will believe almost anything regardless of the evidence.
The problem comes when trying to eliminate an alternative hypothesis. If someone proposes a radical reinterpretation of an ancient battle, it may be sufficient to respond that there is no evidence of that kind of thing happening in any other battle. However, it is much less persuasive to say that we have no evidence of a religion being founded on a mythical crucified Messiah because we have so very little evidence on the founding of any religion.