The first question seems very general and does not specify what “the thing” is. The immediately following question seems to make it clear that “the thing” is the tree which Lehi saw. By changing the determiner the to this, Joseph Smith made the connection between the two questions more obvious, so that the reader would think that the second question was a rephrasing of the first one. Again this is an example of Joseph editing for clarity.
Nonetheless, there is no strong need for this emendation. In all the questions posed by the brothers about their father’s dream (in 1 Nephi 15), the determiner is always the, never this:
There is another possibility here: the initial question may have originally read in the plural— that is, the brothers start out by asking, “What meaneth the things which our father saw in a dream?” They then ask about three specific things that Lehi saw (the tree, the rod of iron, and the river of water), and Nephi gives his response to each one. The addition of “in a dream” in the initial question suggests that this question is a general one. The following three specific questions do not repeat “in the dream”, although they do continue to refer to Lehi as having seen each of these things.
One could counterargue that the singular thing is correct because the associated verb form meaneth ends in the third person singular ending -eth. But in the original Book of Mormon text, the -eth ending was also frequently used with nouns in the plural. In fact, there is one interrogative involving meaneth that has never been edited out of the text:
Thus the appeal to -eth as an historical third person singular ending is not an argument against the original text in 1 Nephi 15:21 as having read “what meaneth the things”. In fact, one could argue that the singular meaneth led scribe 2 of 𝓞 to write the singular thing rather than the plural things.
We have already seen a number of cases where scribe 2 of 𝓞 accidentally dropped the plural s ending, at least momentarily, including one example involving thing:
So we have independent evidence of scribe 2’s tendency to drop the s with things. (For a complete listing of all his examples of losing a final s, see under 1 Nephi 13:23.)
In 1 Nephi 10–15, the text consistently refers to “the things” which Lehi saw in his vision of the tree of life:
The phraseology in all these passages parallels 1 Nephi 15:21 and supports the emended reading “the things which our father saw in a dream”.
There are a couple of places in 1 Nephi 15 where Nephi refers to “the thing which”, but in each case the context definitely implies a singular referent and not all the things Lehi saw in his vision:
Earlier in the text, when Lehi refers to his dream or vision for the first time, we find one more occurrence of the singular thing:
Here Lehi is either referring to his dream or vision as “the thing which I have seen” or he is referring to one specific event in his dream—namely, the acceptance and rejection of the tree of life by his own family (described in 1 Nephi 8:12–18). Of course, it is possible that in 1 Nephi 8:3 we have a primitive error, that the original text actually read in the plural as “the things which I have seen”. The scribe in 𝓞 for 1 Nephi 8 is scribe 3, and there is evidence that this scribe tended to drop plural s’s (see the list under 1 Nephi 4:5). However, both the singular and the plural will work in 1 Nephi 8:3, so there we continue to follow the singular reading of the original manuscript.
In contrast to these examples, the use of the singular in 1 Nephi 15:21 does seem to be quite odd. The plural things makes sense and thus explains the subsequent series of three specific questions the brothers ask (in verses 21, 23, and 26).
Summary: Emend 1 Nephi 15:21 to read “what meaneth the things which our fathers saw in a dream”; there is evidence that scribe 2 of 𝓞 sometimes dropped the plural s, even with things; the more specific questions that follow ask about the meaning of three different things that Lehi saw in his dream.