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Question: Is the Spalding theory of Book of Mormon authorship credible?
The theory requires a second manuscript that doesn't exist, with invented contents, and the invention of a means of getting the alleged manuscript to Joseph Smith via Sidney Rigdon
Modern supporters of the Spalding authorship theory simply ignore the inconvenient fact that the extant Spalding manuscript recovered in the late 19th century bears no resemblance to the Book of Mormon, that it was an unfinished draft, and that no postulated second manuscript has been discovered.
They also ignore the complete lack of any persuasive evidence for contact between Sidney Rigdon and Joseph Smith prior to the Book of Mormon's publication.
Until the purported second manuscript appears, all these critics have is a nonexistent document which they can claim says anything they want. This is doubtlessly the attraction of the "theory" and shows the lengths to which critics will go to disprove the Book of Mormon.
It is interesting to consider that the best explanation such critics can propose requires that they invent a document, then invent its contents, and then invent a means of getting the document to Joseph via Rigdon.
An alleged missing, second Spalding manuscript
The existing Spalding manuscript is obviously unrelated to the Book of Mormon. It is therefore postulated by some that there must exist a second manuscript, despite the fact that the existing manuscript was never completed.
The discovery and publishing of the manuscript put to rest the Spaulding theory for several decades. But in the early 20th century the theory surfaced again, only this time its advocates claimed there was a second Spaulding manuscript that was the real source for the Book of Mormon. However, supporters of the revised Spaulding theory have not produced this second purported manuscript. They do, however, rely upon early works such as a 1908 book written by William Heth Whitsitt called Sidney Rigdon, The Real Founder of Mormonism. The entire book is based upon Whitsitt's initial assumption that Rigdon and Spalding wrote the Book of Mormon. Whitsitt then proceeds to fit the known facts to match that assumption. One of the most amusing parts of the book is the attempt to explain the experience of the Three Witnesses. In Whitsitt's book, Sidney plays the Angel Moroni and the Spalding manuscript itself (the second, undiscovered one) actually plays the part of the gold plates! According to Whitsitt:
It is suspected that Mr. Rigdon was somewhere present in the undergrowth of the forest where the little company were assembled, and being in plain hearing of their devotions he could easily step forward at a signal from Joseph, and exhibit several of the most faded leaves of the manuscript, which from having been kept a series of years since the death of Spaulding would assume the yellow appearance that is well known in such circumstances. At a distance from the station which they occupied the writing on these yellow sheets of paper would also appear to their excited imagination in the light of engravings; Sidney was likewise very well equal to the task of uttering the assurances which Smith affirms the angel was kind enough to supply concerning the genuineness of the "plates" and the correctness of the translation.
See: Solomon Spaulding, Manuscript Found: The Complete Original "Spaulding Manuscript", edited by Kent P. Jackson, (Religious Studies Center, Brigham Young University, 1996). off-site