Hiriam Page said that it would be impossible for Joseph to write the BOM without supernatural power.
Hiram Page, Letter, May 30, 1847 in The Ensign of Liberty of the Church of Christ (Kirtland, Ohio) January 1, 1848, 63
To say my mind was so treacherous that I had forgotten what I saw. To say that a man of Joseph’s ability, who at that time did not know how to pronounce the word Nephi, could write a book of six hundred pages, as correct as the book of Mormon, without supernatural power. . . . [I]t would be treating the God of heaven with contempt, to deny these testimonies, with too many others to mention here.