Witnesses

The Law of Moses required two or three witnesses to convict someone of a crime. Also, commercial transactions and signing of documents of importance was done in the presence of witnesses. As a general rule, a single witness was not competent to attest or testify. (See e.g. Deuteronomy 19:15 and Isaiah 8:2). This was probably also the case for the Nephites, who followed the Law of Moses until the coming of Christ.

For instance, in Alma 9, Alma is charging an entire city with wickedness and prophesy of destruction. The people respond

2 Who art thou? Suppose ye that we shall believe the testimony of one man, although he should preach unto us that the earth should pass away?...
6 And they said: Who is God, that sendeth no more authority than one man among this people, to declare unto them the truth of such great and marvelous things? 

When Amulek enters the scene, the people react differently

And now, when Amulek had spoken these words the people began to be astonished, seeing there was more than one witness who testified of the things whereof they were accused, and also of the things which were to come, according to the spirit of prophecy which was in them. (Alma 10:12)

The ancient Israelite law of witnesses is evidently at play. Apparently, this does not only apply to legal matters but also spiritual matters. One of the reasons Nephi quotes Isaiah, is to call on him as an additional witness to prove that his testimony of the Savior is true.

2 And now I, Nephi, write more of the words of Isaiah, for my soul delighteth in his words. For I will liken his words unto my people, and I will send them forth unto all my children, for he verily saw my Redeemer, even as I have seen him.

3 And my brother, Jacob, also has seen him as I have seen him; wherefore, I will send their words forth unto my children to prove unto them that my words are true. Wherefore, by the words of three, God hath said, I will establish my word. Nevertheless, God sendeth more witnesses, and he proveth all his words. (2 Nephi 11)

To Nephi, God proves his words by calling multiple witnesses. In this case, three eye-witnesses (Isaiah, Nephi and Jacob) who all saw the Redeemer. According to John, we also have three witnesses in heaven.

For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. (1 John 5:7)

The Greek word translated into "bear record" in the KJV means to witness, testify or give evidence. Similar expressions are found in the Book of Mormon, like in 3 Nephi 11:32

I bear record of the Father, and the Father beareth record of me, and the Holy Ghost beareth record of the Father and me

There is another interesting example of the word of God being proven by the law of witnesses. Consider Ether 5:3-4

3 And unto three shall they be shown by the power of God; wherefore they shall know of a surety that these things are true.

4 And in the mouth of three witnesses shall these things be established; and the testimony of three, and this work, in the which shall be shown forth the power of God and also his word, of which the Father, and the Son, and the Holy Ghost bear record—and all this shall stand as a testimony against the world at the last day.

These verses talk about the three witnesses. Verse 4 is a long, convoluted sentence. It starts off by restating the law of witnesses and refers specifically to "these things" which is contextualized by the preceding verses and refers to the Book of Mormon. After sorting this a bit:

• In this work (the coming forth of the Book of Mormon), the power and word of God will be shown forth
• There will be three witnesses to testify of "these things" (the Book of Mormon)
• The Father, Son and Holy Ghost all bear record of the word of God (as found in the Book of Mormon)
• "All this" (above) shall stand as a testimony against the world at the last day

When demonstrating God's power in bringing forth his words in the form of the Book of Mormon in these latter days, there are two sets of witnesses: One on earth (Oliver Cowdery, David Whitmer and Martin Harris) and one in heaven (the Father, the Son and the Holy Ghost). See this post for more context. This should tell us something about both the truthfulness and the importance of the Book of Mormon.

Witnesses of the Book of Mormon plates

The testimonies of the 3 and 8 witnesses in the Book of Mormon are powerful. But I also like to read historical records containing their testimonies reaffirmed in various settings. All in all, I think the testimony from the witnesses is quite strong evidence that the plates actually existed. Critics have tended to be sloppy or dishonest with sources to fit their narrative of a visionary, dream-like experience. However, evaluating all sources and applying source criticism leads to another conclusion. Here are some facts

• There are about 200 historical records containing statements from the witnesses about seeing the plates. These include first, second and third hand accounts
• About 30 accounts are from Oliver Cowdrey, more than 70 accounts from David Whitmer and about 50 from Martin Harris. The rest are from the 8 witnesses. 
• About 8-10 of these 200 accounts (i.e. 5%) include wording like "eye of faith", "vision", "spiritual eyes" or similar. These are the ones critics like to highlight. None of them, however, are first hand accounts, and the source is usually a contemporary critic of the church like Warren Parrish after apostatizing or John A. Clark, a former pastor who considered Joseph Smith a fraud.
• All 11 witnesses publicly reaffirmed their testimony as printed in the beginning of the Book of Mormon and some went out of their way to correct false rumours and newspaper articles claiming that they had denied their testimonies

We cannot get into all the details here, but Richard Lloyd Anderson's book, "Investigating the Book of Mormon witnesses", is a great resource. Anderson also presents at the FairMormon conference here. A few accounts are given below.

It was a clear, open beautiful day, far from any inhabitants, in a remote field, at the time we saw the record, of which it has been spoken, brought and laid before us, by an angel, arrayed in glorious light, [who] ascend [descended I suppose] out of the midst of heaven. Now if this is human juggling—judge ye

-Oliver Cowdery and Martin Harris, in letter dated 29 November 1829 

I saw [the Nephite artifacts] just as plain as I see this bed (striking his hand upon the bed beside him), and I heard the voice of the Lord, as distinctly as I ever heard anything in my life.

-David Whitmer, reported in The Latter-day Saints' Millennial Star Vol. 40 

Gentlemen, do you see that hand? Are you sure you see it? Are your eyes playing a trick or something? No. Well, as sure as you see my hand so sure did I see the angel and the plates.

-Martin Harris, Statement of William M. Glenn, cited in Deseret News, Oct. 2, 1943

Therefore 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

-John Whitmer, "Address To the patrons of the Latter Day Saints' Messenger and Advocate" (March 1836) 

Etc.