As I see it, the single most persuasive secular argument for the authenticity of the founding events of the Restoration isn't actually a single argument or line of evidence at all. It's the fact that no counterexplanation yet proposed for those events accounts for all the relevant data nearly as well as Joseph Smith's own story does.
If his claims aren't true, the fundamental question to be asked regarding Joseph is whether he thought they were or knew they weren't. In other words, was he in some sense "crazy" or was he consciously deceptive?
Let's take those two options in that order.
Joseph Smith might have been hallucinating. That's conceivable. However, raving lunacy scarcely seems to explain the lengthy, coherent and complex Book of Mormon, the Doctrine and Covenants, the Pearl of Great Price, the ordinances of the temple and the creation of The Church of Jesus Christ of Latter-day Saints.
More to the point here, though, is that many others — e.g., the Three Witnesses, the Eight Witnesses, Sidney Rigdon, Mary Whitmer, Emma Smith, Lucy Mack Smith, his siblings William and Katharine, etc. — shared revelatory experiences with him, saw and "hefted" the mysterious objects he possessed, and the like. Were all of them mad, too? Or were they just lying?
Already, the notion that Mormonism can be explained simply as Joseph Smith's hallucination desperately needs one or more major supplements.
Moreover, abundant data about those other co-witnesses demonstrates them to have been sane, sincere, respected and respectable, and, though at considerable personal cost, faithful to their testimonies. And there is no evidence — apart from his claimed revelatory encounters themselves — to indicate that Joseph Smith was prone to hallucination. But to assume in advance that his revelations were hallucinatory is to smuggle into the evidence the very conclusion under dispute, which is a manifest case of what logicians call "circular reasoning."
Suppose, instead, that Joseph was consciously lying. How can this be squared with his surviving personal writings not intended for publication, including journal entries and letters to his wife and children, which breathe sincerity on every page? How does it account for his willingness to suffer greatly (in Liberty Jail, for instance, and during Zion's Camp) and ultimately to die for his claims?
And, again, how would Joseph's supposed dishonesty account for all of those co-witnesses? How, exactly, would it explain their experiences?
Let's suppose that he created bogus plates to fool the Eight Witnesses. (There's not a shred of evidence for this.) How would fake plates account for the other exotic objects seen by the Three Witnesses, let alone explain their vision of an angel and their hearing the confirming voice of God?
So let's assume, instead, that Joseph somehow induced hallucinations in the Three Witnesses. How would their alleged visions account for the much more matter-of-fact mid-day experience of the Eight Witnesses?
At least two very different explanations have to be invoked in order to explain the witness testimonies. Joseph requires the skills of a cunning metalworker (his own, or those of a co-conspirator) and, at the same time, he must be capable either of creating remarkable special effects 150 years before George Lucas or of unerringly selecting people from his small rural community crazy enough to obediently see precisely the visions he needed them to see.
No evidence supports either idea.
While the Book of Mormon was still only a rumor, critics mocked what was sure to be a silly and obvious fraud produced by "that spindle shanked ignoramus Joe Smith." But when the book actually came from the press and proved to be long, complex and remarkably dense, most soon shifted to confident assertions that its source manuscript had been stolen from the classically educated Solomon Spalding, a lapsed Congregationalist minister who had died in 1816.
A tortured historical narrative was fashioned, purporting to explain how Sidney Rigdon, for whatever reason, stole the Spalding manuscript and used it to make Joseph Smith a "prophet." Little or no evidence supports the story, quite a bit of evidence (including very sophisticated recent statistical analysis) renders it deeply improbable and only desperate necessity would ever have given rise to it in the first place. But the Spalding theory nonetheless limps on in certain circles.
Even so, it doesn't even begin to explain the Witnesses, the Doctrine and Covenants, the Pearl of Great Price, and a host of other matters.
Daniel C. Peterson is a professor of Islamic studies and Arabic at BYU, where he also serves as editor in chief of the Middle Eastern Texts Initiative and as director of advancement for the Neal A. Maxwell Institute for Religious Scholarship. He is the founder of MormonScholarsTestify.org. He blogs daily at dcpsicetnon.blogspot.com.