John Whitmer testifies of use of physical existence of plates.
John Whitmer, “Address,” Messenger and Advocate 2, no. 6 (March 1836): 286–87
I desire to testify to all that will come to the knowledge of this address; that I have most assuredly seen the plates from whence the book of Mormon is translated, and that I have handled these plates, and know of a surety that Joseph Smith, Jr. has translated the book of Mormon by the gift and power of God, and in this thing the wisdom of the wise most assuredly has perished