Again, none of these scholars, to the best of my knowledge, takes into account in his investigations the parallel and often conflicting traditions of Shi'ite Islam nor, it goes without saying, does Spencer. Some of the earliest accounts we have are from Shi'ite sources and they raise interesting questions that both complement and challenge scholarly assumptions: for example the early Shi'ite claim that the official Koran, supposedly assembled by the Caliph ‘Uthman from fragments scratched on "palm leaves, the shoulderblades of beasts and the hearts of men", was in fact tampered with and even falsified.  This is the sort of thesis that Spencer should have leapt upon but, like his scholarly sources, his perspective on Islam is narrowly Sunni. He seems unaware, for example, of the magisterial work The Succession to the Prophet (Cambridge, 1997) by Wilferd Madelung, the leading contemporary historian of early Islam, though Madelung's work is a prerequisite to any discussion of Islamic origins. (Surprisingly too, he seems unaware of Stephen J. Shoemaker's brilliant The Death of a Prophet, a book that might have shored up some of his wilder speculations.)  

Second, Spencer has an annoying habit of postulating a hypothesis and then later basing an unfounded conclusion upon it. For example, he opines that the early Quraysh, that Meccan tribe of which Muhammad was a member, may have been originally Christian — whatever "Christian" might have meant in seventh-century Mecca (if indeed, Mecca existed at all); he then later uses this to suggest that the Qur'anic inscriptions of the Dome of the Rock, that magnificent construction built in Jerusalem by the Umayyad Caliph ‘Abd al-Malik in 691, may refer to Jesus rather than to Muhammad or to some vague "praised one" — this based on nothing stronger than the fact that the name Muhammad literally means "the praised one" in Arabic. Here, it seems to me, a quick stroke of Ockham's Razor might have been in order. When Spencer is not clutching at straws, he is tilting at minarets.

Spencer's hypotheses, as it turns out, are no more plausible than the traditional Muslim accounts, and in most instances, decidedly less so.  The lack of written documentation for the period between 632, when the Prophet supposedly died, and 691 when Umayyad coinage or such structures as the Dome of the Rock unambiguously display a Muslim identity — or even the greater gap between 632 and the time of the Prophet's first biographers — proves nothing in itself; it is simply that, a lack of written evidence.  Those who make much of this underestimate or ignore the role of memory in traditional Muslim culture; for us memory is slippery, fallible, elusive. But for those raised in an oral culture, in which the spoken word weighed more than the written, texts committed to memory were deemed superior to those consigned to mere parchment and ink.  From the memorisation of the Koran to the learning of thousands of lines of poetry by heart to the retention of hundreds upon hundreds of traditions, Muslim scholars have always been what Jorge Luis Borges termed "memorious", and to an extent we can hardly grasp. The transmission of detailed lore about Muhammad or early Islam from one scholar to another over generations or even centuries, purely by memory, does not strike me as inherently impossible.  Or might it be that there is something stubborn and uncanny, in me, as in all of us, some memory of mystery, that persuades us to cherish the shadows of all origins?